Allow the use of "cell" and "corner" parameters in spindensity qdens analysis

[aannabe]

Proposed changes

The spindensity estimator can define arbitrary regions in space for density to be sampled. So far, qdens ignored these parameters if they were present in the qmcpack input, resulting in an incorrect *.xsf density output. This PR implements this missing feature in qdens and adds the documentation. This is currently handled for the *.xsf type file only, as I am unfamiliar with the chgcar format.

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

• Bugfix
• New feature
• Documentation changes

Does this introduce a breaking change?

• No

What systems has this change been tested on?

NERSC/Perlmutter: All Nexus tests are passing.

Checklist

• Yes. This PR is up to date with the current state of 'develop'
• Yes. Documentation has been added (if appropriate)

[aannabe]

Added an extra commit that handles cases where there is only a single spin channel in the *.stat.h5 file, such as in the H atom.

[ye-luo]

Test this please