Move twopoint theory

[marcpaterno]

Description

Please include a summary of the change(s). Include which issue is fixed, if this is a bug fix.
Please also include relevant motivation and context. List any new dependencies that are required for this
change.

Fixes # (issue)
Discussion: # (link)

Type of change

Please delete the bullet items below that do not apply to this pull request.

• Refactoring
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

This refactoring contains possible breaking changes because it changes the internal data of TwoPoint, and those internal data are not hidden from users.

Checklist:

The following checklist will make sure that you are following the code style and
guidelines of the project as described in the
contributing page.

• I have run bash pre-commit-check and fixed any issues
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation
• I have 100% test coverage for my changes (please check this after the CI system has verified the coverage)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.7%. Comparing base (95db9c3) to head (c7fc43c).
Report is 12 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #462   +/-   ##
======================================
  Coverage    98.6%   98.7%           
======================================
  Files          53      54    +1     
  Lines        4035    4066   +31     
======================================
+ Hits         3981    4015   +34     
+ Misses         54      51    -3     

Files with missing lines	Coverage Δ
firecrown/likelihood/number_counts.py	91.1% <100.0%> (ø)
firecrown/likelihood/source.py	100.0% <ø> (ø)
firecrown/likelihood/statistic.py	91.1% <ø> (ø)
firecrown/likelihood/two_point.py	99.5% <100.0%> (+0.9%)	⬆️
firecrown/models/two_point.py	100.0% <100.0%> (ø)
firecrown/parameters.py	99.3% <100.0%> (+<0.1%)	⬆️
firecrown/updatable.py	100.0% <100.0%> (ø)