[feature] implement locally spheroidal surface binding potential

[drobnyjt]

Is your feature request related to a problem? Please describe.
Locally planar and locally isotropic (equivalent to spherical) surface binding potentials are just two limiting cases of a generic spheroidal potential. This would take one extra parameter, k, but all it changes is the calculation of the normal vector. This would be a great feature to have when comparing to MD.

Proposed solution
https://doi.org/10.1016/j.nimb.2012.11.022