Basis Selectors and Basis Constraints

[cortner]

I am thinking of retiring the BasisSelectors interface in favour of a BasisConstraints interface. E.g., I could imagine things like the following:

Bsel = Order(3) ∩ TotalDegree(10)
Bsel = Order(5) ∩ WTotalDegree(Dict(...)) ∩ Order(:bond, 1)

etc... no rush, but to me this feels much more intuitive than the fiddling around we are doing right now.

[cortner]

CC @MatthiasSachs @davkovacs (David - it would be useful if you could follow this discussion and chime in to explain what you would find intuitive or not.)