Add SOC & multiple PROCAR parsing functionalities

[kavanase]

This PR implements the PROCAR parsing functionality from easyunfold discussed in #3722; namely the ability to parse PROCARs from spin-orbit coupling (SOC) VASP calculations (i.e. with LSORBIT = True), and the ability to parse multiple PROCAR files together (without wasting time parsing duplicate k-points etc; e.g. if the user has performed split hybrid DFT band structure calculations).

Also adds some further attributes to Procar; is_soc, kpoints, occupancies, eigenvalues and (when is_soc is True) xyz_data which gives the projections data along the x/y/z magnetisation projections.

Tests added for parsing SOC PROCARs, and parsing multiple PROCARs – including when they are SOC and LORBIT = 14 (so phase factors also included and being parsed correctly).

[mkhorton]

Thanks for the PR @kavanase! It looks like we have a merge conflict before we can merge, otherwise this looks great.

[kavanase]

No worries! Yep merge conflict came from some updates in the master branch since opening this, resolved now!

[kavanase]

Just to bump, I think this PR is all ready to go. Fixed the merge conflict that had been introduced with the updated master branch

[janosh]

thanks @kavanase! 👍