Add a whittaker function to do Whittaker-Henderson smoothing and interpolation.

[joa-quim]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 84.21053% with 9 lines in your changes missing coverage. Please review.

Project coverage is 85.97%. Comparing base (ff400ac) to head (9e699a0).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/whittaker.jl	83.92%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1683      +/-   ##
==========================================
+ Coverage   85.95%   85.97%   +0.02%     
==========================================
  Files         160      161       +1     
  Lines       22546    22603      +57     
==========================================
+ Hits        19379    19433      +54     
- Misses       3167     3170       +3     

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