Add attribute to provide a LaTeX representation of a Particle

[namurphy]

This PR adds the Particle.latex_str attribute, which I expect will be used primarily in plotting.

Closes #2640. Would be helpful to include for #2639.

I still have to add tests. I started this PR on an airplane, but hopefully I'll get back to it soon!

[codecov]

Codecov Report

Attention: Patch coverage is 18.18182% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 95.13%. Comparing base (2a6cf7f) to head (39b72a4).
Report is 3 commits behind head on main.

Files	Patch %	Lines
src/plasmapy/particles/particle_class.py	10.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2642      +/-   ##
==========================================
- Coverage   95.18%   95.13%   -0.06%     
==========================================
  Files         103      103              
  Lines        9412     9412              
  Branches     2153     2153              
==========================================
- Hits         8959     8954       -5     
- Misses        276      283       +7     
+ Partials      177      175       -2     

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