WIP: Continue to define Visibility class API

[DanRyanIrish]

This PR carries on from #55 in refining the API of a new Visibilities object. Do not merge until #55 is merged.

[DanRyanIrish]

Thanks for the review @paolomassa. It makes no difference whether the units are defined as arcsec or deg. It just matters that the unit given bad user can be converted to the defined unit. It doesn't mean the value is actually converted to that unit. However, we should probably be consistent throughout the API and use deg or arcsec everywhere. Probably make most sense to use deg as it's the simplest/most common angular unit.

[samaloney]

Thank this looks good, no blockers, few small issues and the tests for the Meta side of things are on the low side.

[codecov]

Codecov Report

Attention: Patch coverage is 81.37255% with 38 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@b79f0bf). Learn more about missing BASE report.

Files	Patch %	Lines
xrayvision/visibility.py	81.37%	38 Missing ⚠️

Additional details and impacted files

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  Lines           ?      829           
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  Hits            ?      740           
  Misses          ?       89           
  Partials        ?        0           

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