Refactor twopoint

[marcpaterno]

This PR will refactor the statistic class TwoPoint to introduce a instance variable TwoPointModel, with the TwoPointModel handling everything to do with theory calculations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.6%. Comparing base (1ec56d9) to head (af87f6b).

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #451   +/-   ##
======================================
  Coverage    98.6%   98.6%           
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  Files          52      53    +1     
  Lines        3888    3905   +17     
======================================
+ Hits         3834    3851   +17     
  Misses         54      54           

Files with missing lines	Coverage Δ
firecrown/generators/two_point.py	100.0% <100.0%> (ø)
firecrown/likelihood/source_factories.py	100.0% <100.0%> (ø)
firecrown/likelihood/two_point.py	98.4% <100.0%> (-0.4%)	⬇️
firecrown/utils.py	100.0% <100.0%> (ø)