Add a listing of interpolator types to the API documentation

[davenquinn]

There are numerous references to interpolator types in the documentation, but there is no authoritative reference on the range of options available. I finally found one by reading through the code. Ideally, the docs would also be a little more centralized in how they discuss the use cases and input parameters for each interpolation scheme.

https://github.com/Loop3D/LoopStructural/blob/6a8d940a0989a046663dcd3dd4ced6f443d895a6/LoopStructural/interpolators/init.py#L49