new class implemented: surface fitting

[imapeartree]

surface fitting: get a masked array, get coefficients of Zernike or polynomial expansions

[lbusoni]

I think most/all the functionality of SurfaceFitter are already in ModalDecomposer. I'll add the documentation to ModalDecomposer and we'll check if it overlaps SurfaceFitter and where to extend it

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.76101% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.07%. Comparing base (e6e71d9) to head (dba3f11).
⚠️ Report is 395 commits behind head on master.

Files with missing lines	Patch %	Lines
arte/utils/surface_fitter.py	73.78%	27 Missing ⚠️
arte/utils/shape_fitter.py	66.66%	1 Missing ⚠️
test/utils/surface_fitter_test.py	98.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   70.88%   71.07%   +0.18%     
==========================================
  Files         135      137       +2     
  Lines        8819     8977     +158     
==========================================
+ Hits         6251     6380     +129     
- Misses       2568     2597      +29     

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