Bspline boundary node contribution bugfix and kernel normalization

[nmbader]

• Fix a bug in Bspline node contributions to the kernel caused by floor function rounding to the incorrect integer.
• Add kernel normalization when irregular node spacing is used to ensure correct partition of unity
• Add unit tests

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.64%. Comparing base (ca43190) to head (cf1b1f8).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
+ Coverage   95.62%   95.64%   +0.01%     
==========================================
  Files          41       41              
  Lines        1120     1125       +5     
==========================================
+ Hits         1071     1076       +5     
  Misses         49       49              

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