Add basic THB spline support

[SouthEndMusic]

Fixes #31.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 43.41085% with 73 lines in your changes missing coverage. Please review.

Project coverage is 87.60%. Comparing base (74b010e) to head (f5e32c5).

Files with missing lines	Patch %	Lines
src/control_points.jl	19.73%	61 Missing ⚠️
src/utils.jl	66.66%	9 Missing ⚠️
src/spline_grid.jl	66.66%	2 Missing ⚠️
src/refinement_matrix.jl	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
- Coverage   94.64%   87.60%   -7.04%     
==========================================
  Files          13       14       +1     
  Lines         821      920      +99     
==========================================
+ Hits          777      806      +29     
- Misses         44      114      +70     

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[SouthEndMusic]

I'm reproducing the example of this paper (p. 6), it's coming along well: