Specialize save_solution_file for AbstractCovariantEquations to support solution_variables which depend on auxiliary variables

[tristanmontoya]

This PR specializes Trixi.save_solution_file for the covariant form to allow for variable conversions that depend on auxiliary variables, and may return a different number of variables than nvariables(equations). Specifically, equations of type AbstractCovariantEquations should use solution_variables(u, aux_vars, equations) instead of solution_variables(u, equations). A default cons2cons(u, aux_vars, equations) = u option has been defined.

[codecov]

Codecov Report

Attention: Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.56%. Comparing base (40800a6) to head (d2dba0d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/callbacks_step/save_solution_covariant.jl	95.00%	1 Missing ⚠️
src/equations/equations.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
+ Coverage   87.49%   87.56%   +0.06%     
==========================================
  Files          17       18       +1     
  Lines        1423     1447      +24     
==========================================
+ Hits         1245     1267      +22     
- Misses        178      180       +2     

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