[FEA] Refactor cubic spline APIs to header-only

[harrism]

Create header-only APIs for the following and refactor existing APIs / tests on top of the header-only API:

• cuspatial::cubicspline_interpolate()
• cuspatial::cubicspline_coefficients()

See https://github.com/rapidsai/cuspatial/blob/branch-22.08/cpp/include/cuspatial/cubic_spline.hpp

[github-actions[bot]]

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[harrism]

Cubic spline functionality removed in #959. Closing.