Cubic splines

[tovrstra]

We should have a C++ CubicSpline class with a Python wrapper, which is derived from the ScalarFunction class, such that it can be used for an efficient evaluation of spherical functions and (spherical harmonic)x(radial function) within a cutoff. Basic features that should be supported:

• Arbitrary positions of the spline grid points should be supported. Don't work with transformations of the x-axis.
• First and Second derivatives of the spline, no inverses.
• Constructor should work with either function values only (and solve for derivatives), or function values and derivatives on the grid.

[tovrstra]

(The current implementation has cubic splines on a uniform grid.)