WIP: Get started on two dimensional regression

[RomeoV]

There is already infrastructure for multi-dimensional regression, except for the polynomial fitting.
This implements basic cubic regression in the two-dimensional case only.
Probably we can write even more general code for arbitrary dimensions, but it's not completely straight forward.

Also, currently the interface is a big mess (should the inputs be Tuples, Vector, SVectors)?

Nonetheless, I'm putting this here. Perhaps I will have more time to refine it - otherwise it may serve as an inspiration to others.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.84%. Comparing base (5286502) to head (e27b5a8).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #99      +/-   ##
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+ Coverage   86.03%   91.84%   +5.80%     
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  Files           2        2              
  Lines         222      233      +11     
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+ Hits          191      214      +23     
+ Misses         31       19      -12     

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