Functions to interpret basis sets: label-to-basis-index and basis-index-to-label

[tovrstra]

(This was part of #146.)

Quoting @PaulWAyers: It would be useful to have a label-to-basis-index and basis-index-to-label functionality, similar to PySCF. It would not need to be implemented exactly like this, but it would be nice to have functionality that, for a given basis function (say, the 17th) would tell you its type, contraction coefficients, exponents, center etc. (e.g., that it was the 3rd dz^2 basis function on the second atomic center). Similarly, it would be nice to be able to ask for a specific type of basis function (e.g., the 3rd dz^2 on a specified atom) and return the basis index.