refactor: normalize variable names across caustics objects

[ConnorStoneAstro]

Update to get the naming conventions in line. Now all lenses defined with an Einstein radius use Rein. all lenses defined with a mass use mass. Other radii are defined with R___ such as the core radius which is Rc and scale radius which is Rs.

[codecov]

Codecov Report

Attention: Patch coverage is 95.50562% with 4 lines in your changes missing coverage. Please review.

Project coverage is 90.35%. Comparing base (264b300) to head (165a17d).

Files with missing lines	Patch %	Lines
src/caustics/lenses/tnfw.py	70.00%	3 Missing ⚠️
src/caustics/lenses/nfw.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #302      +/-   ##
==========================================
+ Coverage   90.31%   90.35%   +0.03%     
==========================================
  Files          55       55              
  Lines        2281     2280       -1     
==========================================
  Hits         2060     2060              
+ Misses        221      220       -1     

Flag	Coverage Δ
unittests	90.35% <95.50%> (+0.03%)	⬆️

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