Initial support for tensor-product splines

[jipolanco]

To do:

• add and fix tests
• document tensor-product splines
• add multidimensional interpolations
• support derivatives and integrals?

[codecov-commenter]

Codecov Report

Merging #41 (60fa41a) into master (60fa41a) will not change coverage.
The diff coverage is n/a.

❗ Current head 60fa41a differs from pull request most recent head 86dfd9d. Consider uploading reports for the commit 86dfd9d to get more accurate results

@@           Coverage Diff           @@
##           master      #41   +/-   ##
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  Coverage   95.21%   95.21%           
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  Files          26       26           
  Lines        1650     1650           
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  Hits         1571     1571           
  Misses         79       79           

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