Supporting BSE

[azadoks]

Currently, BasisSets are represented and stored by aiida-basis as SingleFileDatas in AiiDA.
This implementation makes sense for numerical orbitals like OpenMX's PAOs because

0. The basis is used in practice as a single file per element
1. There is a single file-format for the basis
2. The contents of the basis don't need to be modified

However, this is not true in general.
Specifically, to support basis sets from the Basis Set Exchange, this architecture needs to be rethought.

I've opened this issue as a place to discuss what a more general implementation would look like.

[azadoks]

My naïve suggestion would be to:

• Rename BasisData to BasisFileData; it will be used to represent/store file-based bases (one element per basis)
• Create a new base class BasisData used for storing the data (not file) of a basis (one element per basis)
• Modify BasisSet to support both BasisFileData types and BasisData types

Another note is on nomenclature, currently I am calling a basis set which corresponds to a element/species a "basis" while a collection of basis sets, e.g. STO-3G for all available elements/species, a "basis set".
As I don't have much experience with how these things are called in the community, perhaps this should be changed.
Does it make more sense to call the data for a single element a "basis set" and the data for a collection of elements a "basis set family"?