Nexus: Small fix to excitation checks

[mcbennet]

Proposed changes

This PR provides a fix to an off-by-one error when checking that the einspline files contain the correct excitation.

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?

ALCF Theta

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
• N/A. 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)

[ye-luo]

Test this please