Add least squares approximation

[JoshuaLampert]

Allow to interpolate, where centers of basis functions differ from the data sites to perform a least squares approximation.

TODO:

• add example
• fix functions using nodeset(itp). Need nodeset.centers now?!
• double-check with polynomials
• double-check for solving PDEs with collocation (stationary and time-dependent): postponed for now, see Add possibility to use different node set for centers to solve PDEs #97
• add test
• docstring
• docs (tutorial?): postponed (will be together with regularization), see Add tutorial on noisy data #95

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.61%. Comparing base (56f338e) to head (c97eea4).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
+ Coverage   98.59%   98.61%   +0.02%     
==========================================
  Files          40       41       +1     
  Lines        1137     1155      +18     
==========================================
+ Hits         1121     1139      +18     
  Misses         16       16              

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