ENH: Add a utility for finding sweep number keys

[mgrover1]

Add a utility to find the sweep_n keys in a given datatree. This will help with downstream applications, especially with Py-ART, and since this information will be stored in the core data model.

• [x] Tests added
• [x] Changes are documented in history.md

[codecov[bot]]

Codecov Report

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

Project coverage is 91.33%. Comparing base (0145954) to head (5945ad2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #167 +/- ## ========================================== + Coverage 91.30% 91.33% +0.02% ========================================== Files 21 21 Lines 3956 3969 +13 ========================================== + Hits 3612 3625 +13 Misses 344 344 ``` | [Flag](https://app.codecov.io/gh/openradar/xradar/pull/167/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openradar) | Coverage Δ | | |---|---|---| | [notebooktests](https://app.codecov.io/gh/openradar/xradar/pull/167/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openradar) | `80.47% <7.69%> (-0.24%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/openradar/xradar/pull/167/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openradar) | `89.64% <100.00%> (+0.03%)` | :arrow_up: | 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=openradar#carryforward-flags-in-the-pull-request-comment) to find out more.

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[mgrover1]

I'll add that in!! 👍

[mgrover1]

@kmuehlbauer good to merge?