add correct indexing for SpinDensityNew output

[camelto2]

Please review the developer documentation
on the wiki of this project that contains help and requirements.

Proposed changes

small bugfix for spin density output to hdf5. Fixes #4533

Note: this only fixes the problem. Actual testing to catch bugs like this and a better design which doesn't rely on ObservableHelper will follow this up (see #4540)

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

Delete the items that do not apply

• Bugfix

Does this introduce a breaking change?

• No

What systems has this change been tested on?

M1 mac

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)

[ye-luo]

Test this please