Adding dummy time index conversion function for warming levels

[claalmve]

Description of PR

Summary of changes and related issue For several tools in our codebase (and for many external library calculations), there is a need for a dimension named time in a given DataArray for calculations. However, warming levels code removes this time index and replaces it with a hours/days/months_from_center dimension, since that is more accurate name than just time. This also does create some challenges with computing on this DataArray, since the hours/days/months_from_center dimension is filled with timedeltas and no longer timestamps. So, this PR is focused on including a helper function in utils that allows a user to toggle from WL indexing to a dummy time-based indexing to allow for potential metric calculations to still occur as desired.

This function is aimed to be used in vulnerability.py and warming_levels_approach.ipynb (included in https://github.com/cal-adapt/cae-notebooks/pull/103)

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules