Add property access by index, not just by shell

[matt-chan]

Stijn has suggested we add access to properties of the basis functions (ie exponent, center, normalization, angular momentum(?), etc) by index. So for example, the properties of the [5,2] element of the overlap matrix. Right now, you can infer what they are if you know how the subshell, the basis ordering, and the attributes of GBasis are implemented. It's a lot of internal algorithmic knowledge that I also think we shouldn't be expecting casual users to know.

This should be implemented as python utils I think. It should be fairly easy to do as well, possibly a good intro project into GBasis.