Cubic spline interpolation

This is *almost*, but not quite, ready.

* [x] Some tests, mainly related to derivatives, still fail. I suspect I might have been too tired when I derived the polynomials for `interp_gcoefs_1d` and `interp_hcoefs_1d` - will look into it.
* ~~[ ] More boundary conditions - currently only `BCna`, `BCnan` and `BCnil` are supported~~
* [x] Documentation