WIP: Add option to subtract a reference state in SaveSolutionCallback

[tristanmontoya]

This PR allows one to subtract a reference solution state (e.g. a steady base flow or exact solution) when using SaveSolutionCallback, allowing one to more easily plot perturbations using Trixi2Vtk. This is used in the warm bubble example to save the solution as a perturbation of a hydrostatically balanced base state.

[github-actions]

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Created with ❤️ by the Trixi.jl community.

[codecov]

Codecov Report

Attention: Patch coverage is 26.08696% with 34 lines in your changes missing coverage. Please review.

Project coverage is 93.85%. Comparing base (4a81e00) to head (3b86205).

Files with missing lines	Patch %	Lines
...es/structured_2d_dgsem/elixir_euler_warm_bubble.jl	0.00%	27 Missing ⚠️
src/callbacks_step/save_solution.jl	53.85%	6 Missing ⚠️
src/callbacks_step/save_solution_dg.jl	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2131      +/-   ##
==========================================
- Coverage   96.36%   93.85%   -2.50%     
==========================================
  Files         477      477              
  Lines       37753    37778      +25     
==========================================
- Hits        36378    35456     -922     
- Misses       1375     2322     +947     

Flag	Coverage Δ
unittests	93.85% <26.09%> (-2.50%)	⬇️

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