Create a class for nuclear reactions

[namurphy]

[to be added later]

First part of #2535.

[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.71%. Comparing base (93bf7ae) to head (fca509d).

❗ Current head fca509d differs from pull request most recent head 7049264

Please upload reports for the commit 7049264 to get more accurate results.

Files with missing lines	Patch %	Lines
plasmapy/particles/interactions.py	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2537      +/-   ##
==========================================
+ Coverage   95.12%   96.71%   +1.58%     
==========================================
  Files         107      105       -2     
  Lines        9626     9395     -231     
  Branches     2230        0    -2230     
==========================================
- Hits         9157     9086      -71     
- Misses        280      309      +29     
+ Partials      189        0     -189     

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[namurphy]

Make the ABC private while we are figuring out the interface?