Remove duplicated instances of closest_index

[moprak-nrel]

Summary

The closest_index function seems to have been copy pasted into three different places, consolidating them under one common header file. Moving another unnamed namespace function into the new common header file as well.

Pull request type

Please check the type of change introduced:

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [x] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

Checklist

The following is included:

• [ ] new unit-test(s)
• [ ] new regression test(s)
• [ ] documentation for new capability

This PR was tested by running:

• the unit tests
  • [ ] on GPU
  • [x] on CPU
• the regression tests
  • [ ] on GPU
  • [ ] on CPU