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 82.60%. Comparing base (795feb0) to head (449149f).

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   82.43%   82.60%   +0.17%     
==========================================
  Files          21       22       +1     
  Lines         905      943      +38     
  Branches       52       57       +5     
==========================================
+ Hits          746      779      +33     
- Misses        155      160       +5     
  Partials        4        4              

Flag	Coverage Δ
unittests	82.60% <86.84%> (+0.17%)	⬆️

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