Add Spin-orbit coupling as a main button

[edan-bainglass]

Make it a main button below the one for magnetism.

• Name it: "Spin-orbit coupling": "Off" and "On" like magnetism, with Off by default

• Make sure this also goes into the default label (add "with spin orbit")

[edan-bainglass]

@AndresOrtegaGuerrero do you already have a PR for this? I know you've already handled the label in #954. If you're not already handling moving the widget to basic, I got it.

[edan-bainglass]

@cpignedoli @giovannipizzi thoughts? (ignore the capitalization ("Electronic Type") - that will be addressed throughout in a separate PR)

[giovannipizzi]

lgtm!

[cpignedoli]

Super @edan-bainglass. We will probably have to add in a different PR some warnings/info on the pseudopotentials to be used and on the fact that one should consider doing geoopt without SOC in "some cases". What do you think?