Nexus: Radial density of requested species only

[mcbennet]

Proposed changes

Allows users to calculate radial density of subset of species -- passed as an argument (species) to radial_density.
This can speed up workflows, for instance, when user is only interested in magnetic moment of one (or few) species.
Significant speed-up when user is resampling magnetic moment (or charge) of one (or few) species -- in this case radial_density will generally be called hundreds of times.

nxs-test yields 100% success rate.

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

• New feature

Does this introduce a breaking change?

• No

What systems has this change been tested on?

Laptop

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'
• N/A. 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]

This looks fine.

[ye-luo]

Okay to test