Composition - UserDict subclassing

[SamG-01]

Changes the Composition class to subclass UserDict, allowing for direct key-value access of nuclei by string (e.g. comp["he4"]) or by Nucleus object (e.g. comp[Nucleus("he4")] or comp.X[Nucleus("he4")]. This also allows for dictionary operations such as for nuc, x in comp.items(): and len(comp). Also adds comp.A and comp.Z getters for dictionaries of the molar masses and charges.

[yut23]

I really like __getitem__/__setitem__ being able to handle strings; that'll make interactive use a lot nicer. We can probably remove a lot of the boilerplate functions by inheriting from collections.UserDict.

[SamG-01]

With this, certain methods like set_nuc will become obsolete. Should these be kept in the class for compatibility with existing code?

[SamG-01]

In addition to subclassing UserDict, I also added a getter and setter for a comp.X property for backwards compatibility, as well as comp.A and comp.Z getters for dictionaries of the molar masses and charges (such that comp[n].A = comp.A[n]). The methods comp.get_nuclei() and comp.set_nuc(name, xval) are now superseded by list(comp.keys()) and comp[name] = xval respectively, but are left in for compatibility with existing code for the time being.

[yut23]

Can you add a quick test to make sure all the different indexing variations work the same? (comp[str], comp[Nucleus], comp.X[Nucleus])
I'll make a follow-up PR to update the usages elsewhere in the code.

[yut23]

Pylint warnings are unrelated, from the new 3.2.0 version