Updates to (intermediate) terms

[mortenpi]

With #64 in mind, but also other edits.

• I added .nu as a property to IntermediateTerm. I think ideally we'd provide such fallbacks in all the cases where we're using Unicode as fields.

• I am documenting the various fields as the official API to access e.g. the L & S quantum numbers of a term etc.

• Probably not worth it, but one could rename Term -> LSTerm, to make it more explicit. Theoretically, one could also have LSJTerm (or should it be LSJLevel then?).

[mortenpi]

I'd say this is good to go from my side.