Implement collision source terms for multi-ion MHD

[amrueda]

This PR introduces collision source terms for multi-ion MHD, accompanied by detailed documentation and a physically relevant test case based on the following reference:

• Ghosh, D., Chapman, T. D., Berger, R. L., Dimits, A., & Banks, J. W. (2019). A multispecies, multifluid model for laser–induced counterstreaming plasma simulations. Computers & Fluids, 186, 38-57.

The Figure shows the temperature of two ion species for the second frictional slow-down test described in the reference above, as computed with examples/tree_2d_dgsem/elixir_mhdmultiion_collisions.jl and source_terms = source_terms_collision_ion_ion.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.91%. Comparing base (0e9eb6a) to head (c8b4bf4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2213      +/-   ##
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+ Coverage   96.90%   96.91%   +0.01%     
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  Files         491      492       +1     
  Lines       40093    40201     +108     
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+ Hits        38852    38960     +108     
  Misses       1241     1241              

Flag	Coverage Δ
unittests	96.91% <100.00%> (+0.01%)	⬆️

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[JoshuaLampert]

I didn't check the math, but overall this looks good to me.