Add searchable spheroid

This is an idea that I discussed offline with @Mattijs. Having spheroid as a shape could be useful for external surroundings etc. Found some ideas, but equally importantly I have someone willing to implement it (@victorOle). The first efforts look good, but would likely make most sense to merge into searchableSphere and treat the uniform radii as a special optimized case.

Largely able to work from Geometric Tools documentation (CC-BY 4.0), but needs rework for the OpenFOAM framework