Rename named tuple in febus module

[d-chambers]

Description

The public-named tuple in the febus.core module causes issues with the doc build because the inspect module is not able to get the source file for it. This PR just renames it to be private to circumvent the issue.

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 99.53%. Comparing base (76d4583) to head (4f0bb3b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #371 +/- ## ======================================= Coverage 99.53% 99.53% ======================================= Files 97 97 Lines 7596 7596 ======================================= Hits 7561 7561 Misses 35 35 ``` | [Flag](https://app.codecov.io/gh/DASDAE/dascore/pull/371/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/371/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE) | `99.53% <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.

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