Write data

[benegee]

This PR adds new methods trixi_load_conservative_vars and trixi_save_conservative_vars to read and write conservative variables by copying data. Moreover, a method trixi_get_data_pointer is added, which returns a pointer to Trixi's internal simulation data array. Thus no data needs to be copied, but it is potentially more risky.

Both approaches can be used to modify the conservative variables. This is demonstrated by changing the concentration of a passive tracer during a simulation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.15%. Comparing base (3e1d8e6) to head (d97e743).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
+ Coverage   95.62%   96.15%   +0.52%     
==========================================
  Files          23       25       +2     
  Lines        1097     1273     +176     
  Branches       63       74      +11     
==========================================
+ Hits         1049     1224     +175     
- Misses         44       45       +1     
  Partials        4        4              

Flag	Coverage Δ
unittests	96.15% <100.00%> (+0.52%)	⬆️

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[sloede]

Very nice extension, I just have two naming suggestions/comments

[sloede]

I did only a cursory check since it seems to be a lot of regex search and replace. But after this, I think a new breaking release would be in order :-)