Support for effective core potentials (ECPs)

[PaulWAyers]

Currently we don't support ECPs in gbasis. It would be nice to do that. Given the (new) strategy of falling back on a non-Python implementation where available as necessary, it may be easier to do that. There is an open-source library for ECP integrals (C++). https://github.com/robashaw/libecpint

For now I think this is low(ish) priority but this is a placeholder so we don't forget. It's something that I think can be supported relatively easily when/if it is needed.