WIP: Vasp RPA optics

[ajjackson]

VASP can calculate frequency-dependent dielectric functions within the random-phase approximation using ALGO=CHI. This doesn't seem to be recognised in Pymatgen so it may be simpler to add a special XML parser for these modes. (Not sure what you get from the GW and BSE modes, maybe more things to be managed by this parser?)

In the present implementation this requires a --code=vasp-rpa argument to sumo-optplot and is built on the experimental Questaal branch to ensure compatibility with this option.

Goals for this PR:

• Import frequency-dependent dielectric matrices from ALGO=CHI output
• Include some sample data
• Test the sample data

Long-term considerations:

• Look at GW and BSE outputs, see how similar they are
• Add support if this is simple

[ajjackson]

Codacy is grumpy that we are using the standard library XML parser, and wants us to add a dependency to our project for a more secure parser.

The thing is, we already use Pymatgen to parse Vasp XML files, and Pymatgen also uses the (insecure) ElementTree parser. So this isn't actually any less secure than the existing features, it's just easier for Codacy to notice.

It's not that I'm opposed to adding a dependency for safer parsing, bit it seems a bit pointless unless we get it into Pymatgen as well.