Make copy_to_coupled_boundary! threaded

[efaulhaber]

When running a simulation with 100k DOFs on my laptop with 6 threads (the difference will be much bigger on more threads):
Before (with #1978 and #1979):

────────────────────────────────────────────────────────────────────────────────────────────────────
Trixi.jl simulation finished.  Final time: 1.0368e6  Time steps: 2736 (accepted), 2736 (total)
────────────────────────────────────────────────────────────────────────────────────────────────────

 ───────────────────────────────────────────────────────────────────────────────────────
               Trixi.jl                        Time                    Allocations      
                                      ───────────────────────   ────────────────────────
           Tot / % measured:               35.9s /  63.7%           2.19GiB /  99.8%    

 Section                      ncalls     time    %tot     avg     alloc    %tot      avg
 ───────────────────────────────────────────────────────────────────────────────────────
 copy to coupled boundaries    13.7k    13.9s   60.6%  1.01ms   1.91GiB   87.7%   147KiB
 rhs!                          82.1k    7.87s   34.4%  95.8μs   89.3MiB    4.0%  1.11KiB
   volume integral             82.1k    3.67s   16.0%  44.7μs   22.6MiB    1.0%     288B
   interface flux              82.1k    1.51s    6.6%  18.4μs   26.5MiB    1.2%     339B
   surface integral            82.1k    826ms    3.6%  10.1μs   21.3MiB    1.0%     272B
   reset ∂u/∂t                 82.1k    728ms    3.2%  8.87μs   6.36KiB    0.0%    0.08B
   boundary flux               82.1k    643ms    2.8%  7.83μs     0.00B    0.0%    0.00B
   Jacobian                    82.1k    423ms    1.8%  5.15μs   18.9MiB    0.8%     241B
   ~rhs!~                      82.1k   65.8ms    0.3%   801ns   5.14KiB    0.0%    0.06B
   source terms                82.1k   1.04ms    0.0%  12.6ns     0.00B    0.0%    0.00B
 calculate dt                  2.74k    658ms    2.9%   241μs   34.5MiB    1.5%  12.9KiB
 I/O                              50    482ms    2.1%  9.65ms    151MiB    6.8%  3.03MiB
   save solution                 294    480ms    2.1%  1.63ms    150MiB    6.7%   522KiB
   ~I/O~                          50   1.92ms    0.0%  38.3μs    928KiB    0.0%  18.6KiB
   save mesh                      49    139μs    0.0%  2.84μs    602KiB    0.0%  12.3KiB
   get element variables         294   92.8μs    0.0%   316ns     0.00B    0.0%    0.00B
   get node variables            294   5.25μs    0.0%  17.8ns     0.00B    0.0%    0.00B
 ───────────────────────────────────────────────────────────────────────────────────────

This PR (with #1978 and #1979):

────────────────────────────────────────────────────────────────────────────────────────────────────
Trixi.jl simulation finished.  Final time: 1.0368e6  Time steps: 2736 (accepted), 2736 (total)
────────────────────────────────────────────────────────────────────────────────────────────────────

 ───────────────────────────────────────────────────────────────────────────────────────
               Trixi.jl                        Time                    Allocations      
                                      ───────────────────────   ────────────────────────
           Tot / % measured:               26.4s /  42.6%           2.31GiB /  99.9%    

 Section                      ncalls     time    %tot     avg     alloc    %tot      avg
 ───────────────────────────────────────────────────────────────────────────────────────
 rhs!                          82.1k    5.83s   51.8%  71.0μs   88.9MiB    3.8%  1.11KiB
   volume integral             82.1k    2.83s   25.2%  34.5μs   22.5MiB    1.0%     288B
   interface flux              82.1k    1.04s    9.2%  12.6μs   26.3MiB    1.1%     336B
   boundary flux               82.1k    595ms    5.3%  7.25μs     0.00B    0.0%    0.00B
   surface integral            82.1k    551ms    4.9%  6.71μs   21.3MiB    0.9%     272B
   reset ∂u/∂t                 82.1k    455ms    4.0%  5.54μs     0.00B    0.0%    0.00B
   Jacobian                    82.1k    293ms    2.6%  3.57μs   18.8MiB    0.8%     240B
   ~rhs!~                      82.1k   59.7ms    0.5%   727ns   5.14KiB    0.0%    0.06B
   source terms                82.1k   1.03ms    0.0%  12.5ns     0.00B    0.0%    0.00B
 copy to coupled boundaries    13.7k    4.32s   38.4%   316μs   2.04GiB   88.4%   156KiB
 calculate dt                  2.74k    713ms    6.3%   261μs   34.5MiB    1.5%  12.9KiB
 I/O                              50    395ms    3.5%  7.90ms    151MiB    6.4%  3.03MiB
   save solution                 294    393ms    3.5%  1.34ms    150MiB    6.3%   522KiB
   ~I/O~                          50   1.86ms    0.0%  37.2μs    928KiB    0.0%  18.6KiB
   save mesh                      49    137μs    0.0%  2.79μs    602KiB    0.0%  12.3KiB
   get element variables         294   96.0μs    0.0%   327ns     0.00B    0.0%    0.00B
   get node variables            294   8.92μs    0.0%  30.3ns     0.00B    0.0%    0.00B
 ───────────────────────────────────────────────────────────────────────────────────────

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.16%. Comparing base (5398b22) to head (58707b1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1981      +/-   ##
==========================================
- Coverage   96.16%   96.16%   -0.00%     
==========================================
  Files         460      460              
  Lines       36958    36958              
==========================================
- Hits        35539    35538       -1     
- Misses       1419     1420       +1     

Flag	Coverage Δ
unittests	96.16% <100.00%> (-<0.01%)	⬇️

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[SimonCan]

Looks good to me.

[sloede]

LGTM once the final comment is resolved. Thanks for your efforts, @efaulhaber!

[ranocha]

Thanks