Nexus: Extend GCTA to SOC Calculations

[aannabe]

Proposed changes

Previously the grand-canonical twist-averaging (GCTA) implementation was hard-coded for collinear systems with spin up and down electrons only. This PR extends it to accept noncollinear systems for SOC calculations. It is mostly achieved by ignoring the down spin where necessary, similar to the qmcpack input.

I also fixed a minor bug (line 156), which erroneously asserted that normalization factor for k-points must be integer. This worked fine for even k-mesh grids, but errors out in odd k-mesh grids, as found by @camelto2 .

What type(s) of changes does this code introduce?

• New feature
• Bugfix
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)

Does this introduce a breaking change?

• No

What systems has this change been tested on?

Tested on OLCF/Andes using fcc Au system (only input generation using Nexus, no QMC runs).

Checklist

• Yes. This PR is up to date with current the current state of 'develop'

[prckent]

What did you test this on and how did you test it?

[aannabe]

At this point I tested that it successfully generates the QMC inputs on fcc gold. I also tested that the previous collinear is intact by running nspin = 1 and nspin = 2 on gold. Similarly, Al and Fe collinear systems work without issues. Again, these tests were done only at Nexus level and by hand. It would be good to add some GCTA tests in future.

[prckent]

Test this please