Allow ions instead of atoms in CIF files

[DorianDepriester]

Is your feature request related to a problem? Please describe.
In some CIF files, atoms are given by their ionic names (e.g. Fe2+ instead of Fe). This is (sometimes) the case for generated files from The Materials Project. This actually results in failure of get_element_id_from_string().

Describe the solution you'd like
A simple regexp to automatically trim the atom names.

Describe alternatives you've considered
The user can edit the CIF file to remove the exponents.

[CSSFrancis]

@DorianDepriester I think ions result in slightly different atomic scattering factors.

I got fairly far on adding them but just never did. If this is something you are interested in I can prioritize this!

[DorianDepriester]

From my point of view, it would indeed be an interesting feature 😉

[CSSFrancis]

From my point of view, it would indeed be an interesting feature 😉

I'll see if I can make the PR later this week and we can think about making a new release!