fix correlate doc build

[d-chambers]

Description

This PR fixes the issues with the documentation build from the recent correlate changes.

It also keeps the new correlate coordinate in compatible units rather than forcing a conversion to float. For example, if the time coord was a datetime64 the "lag_time" coordinate will now be a timedelta64 coordinate.

Checklist

I have (if applicable):

• [ ] referenced the GitHub issue this PR closes.
• [ ] documented the new feature with docstrings or appropriate doc page.
• [ ] included a test. See testing guidelines.
• [ ] your name has been added to the contributors page (docs/contributors.md).
• [ ] added the "ready_for_review" tag once the PR is ready to be reviewed.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.84%. Comparing base (9bbae76) to head (6885541).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #402 +/- ## ======================================= Coverage 99.84% 99.84% ======================================= Files 109 109 Lines 8788 8788 ======================================= Hits 8774 8774 Misses 14 14 ``` | [Flag](https://app.codecov.io/gh/DASDAE/dascore/pull/402/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/DASDAE/dascore/pull/402/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE) | `99.84% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.