[WIP] Support for electron density grid representations

This PR will implement:
* estimating atomic positions from a density map
* computing finite scattering from a density map
* conversions between square grids and spherical harmonic representations

These items will provide a lot of support for working with electron density maps as well as atomic representations.

@dermen likely nothing works yet, but watch this branch -- in a day or two we'll be ready to simulate your NP!