Refactor pint.fitter

[abhisrkckl]

Reduce code duplication in pint.fitter, e.g., between WLSFitter and DownhillWLSFitter.

A side benefit of this is that it simplifies the Fitter classes, making it easier to implement #1660.

I accidentally merged #1883 into this, so that should be merged first.

[codecov]

Codecov Report

Attention: Patch coverage is 96.52174% with 4 lines in your changes missing coverage. Please review.

Project coverage is 69.79%. Comparing base (23870cf) to head (b38db89).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/pint/fitter.py	97.97%	2 Missing ⚠️
src/pint/models/timing_model.py	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1884      +/-   ##
==========================================
- Coverage   69.84%   69.79%   -0.06%     
==========================================
  Files         110      110              
  Lines       25740    25674      -66     
  Branches     4072     4056      -16     
==========================================
- Hits        17977    17918      -59     
+ Misses       6627     6622       -5     
+ Partials     1136     1134       -2     

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