Add basis_spline stateful transform.

[matthewwardrop]

This patch adds an implementation of basis splines into formulaic. It's performance beats that of scipy's splev for our use cases, and so we do not use it here.

This patch highlighted a few deficiencies elsewhere in the stack (such as implicit sparse matrix encoding), and this needs to be unit tested, and so this will sit here for a while until those issues are sorted.

[codecov]

Codecov Report

Merging #21 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #21   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        31    +1     
  Lines         1233      1286   +53     
=========================================
+ Hits          1233      1286   +53     

Flag	Coverage Δ
#unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
formulaic/materializers/transforms/basis_spline.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 93be547...b367270. Read the comment docs.