Nexus: Add small fix for custom band plotting

[mcbennet]

Proposed changes

Fixing small issue in plot_bandstructure when a custom k-path is specified.

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

• Bugfix

Does this introduce a breaking change?

• No. All nxs-test tests pass

What systems has this change been tested on?

Laptop

Checklist

• 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
• N/A. Documentation has been added (if appropriate)

[qmc-robot]

Can one of the admins verify this patch?

[jtkrogel]

LGTM

[markdewing]

Test this please