Refactor the internal numerical_inverse API

[WilliamJamieson]

This performs an internal refactor of the numerical_inverse API, so that the ideas presented in #566 can be incorporated into the general API refactor.

[codecov]

Codecov Report

Attention: Patch coverage is 91.70507% with 18 lines in your changes missing coverage. Please review.

Project coverage is 86.10%. Comparing base (c6ee47f) to head (b2b934b).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
gwcs/wcs/_pipeline.py	74.64%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #571      +/-   ##
==========================================
+ Coverage   86.04%   86.10%   +0.06%     
==========================================
  Files          27       28       +1     
  Lines        4048     4081      +33     
==========================================
+ Hits         3483     3514      +31     
- Misses        565      567       +2     

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