Add load_timestep! for restart setup

[ArseniyKholod]

In every restart setup we have to get the last time index and set the number of accepted steps in way:

integrator.iter = load_timestep(restart_filename)
integrator.stats.naccept = integrator.iter 

It would be useful to combine these lines into one function load_timestep!(integrator, restart_filename)

[github-actions]

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[sloede]

Thanks a lot, this looks already very good! Just two minor comments to resolve, then this is GTM!

[sloede]

LGTM, thanks a lot!

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (27d4fd1) 92.94% compared to head (74e42bc) 92.94%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1614   +/-   ##
=======================================
  Coverage   92.94%   92.94%           
=======================================
  Files         416      416           
  Lines       34065    34068    +3     
=======================================
+ Hits        31661    31664    +3     
  Misses       2404     2404           

Flag	Coverage Δ
unittests	92.94% <100.00%> (+<0.01%)	⬆️

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Files Changed	Coverage Δ
src/Trixi.jl	43.48% <ø> (ø)
src/callbacks_step/save_restart.jl	91.53% <100.00%> (+0.45%)	⬆️

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