Nexus: Allow non spin-specific occupations in case of noncollinear

[mcbennet]

Proposed changes

Allow user to run with noncollin as true and use a non spin-specific occupation.

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

Small change

• Other (please describe): Small change

Does this introduce a breaking change?

• nxs-test is failing on rmg_input before and after my change?

What systems has this change been tested on?

Summit

Checklist

Update the following with a yes where the items apply. If you're unsure about any of them, don't hesitate to ask. This is
simply a reminder of what we are going to look for before merging your code.

• Yes. This PR is up to date with current the current state of 'develop'
• Yes. Code added or changed in the PR has been clang-formatted
• No. This PR adds tests to cover any new code, or to catch a bug that is being fixed
• No. Documentation has been added (if appropriate)

[qmc-robot]

Can one of the admins verify this patch?

[jtkrogel]

I recommend we track down the failing test first and then merge following a PR with the fix.

[jtkrogel]

Please open a new issue detailing the test failure.

[ye-luo]

#2958 was opened for tracking the test failure.

[prckent]

OK to test