RFC: raw data access

[benegee]

This PR adds a new method trixi_get_data_pointer, which returns a pointer to Trixi's internal simulation data array.

This can be used to modify the conservative variables. Combined with poor man's tracer advection in #199 and trixi_get_simulation_time it should be possible to implement the evolution of tracer concentrations, e.g. based on the solution of some chemistry ODE.

Of course this is also dangerous.

[codecov]

Codecov Report

Attention: Patch coverage is 86.84211% with 5 lines in your changes missing coverage. Please review.

Project coverage is 95.33%. Comparing base (0e0ea3d) to head (9dec2df).

Files with missing lines	Patch %	Lines
LibTrixi.jl/src/api_c.jl	25.00%	3 Missing ⚠️
LibTrixi.jl/src/api_jl.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
- Coverage   95.62%   95.33%   -0.30%     
==========================================
  Files          23       24       +1     
  Lines        1097     1135      +38     
  Branches       63       68       +5     
==========================================
+ Hits         1049     1082      +33     
- Misses         44       49       +5     
  Partials        4        4              

Flag	Coverage Δ
unittests	95.33% <86.84%> (-0.30%)	⬇️

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