Add index_by_label method to Visibilities class

[DanRyanIrish]

DO NOT MERGE UNTIL #64 is merged.

This PR adds an index_by_label method to Visibilities which allows a subset of visibilities to be copied into a new Visibilities instance based on their Visibilities.meta.vis_labels values. It also adds a __getitem__ method so it can be sliced with the standard slicing API.

This method could potentially be added to the VisibilitiesABC, but I don't think that discussion needs to hold up this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@adfb913). Learn more about missing BASE report.

Files	Patch %	Lines
xrayvision/visibility.py	90.62%	3 Missing ⚠️

Additional details and impacted files

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##             main      #65   +/-   ##
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  Coverage        ?   89.04%           
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  Files           ?        7           
  Lines           ?      904           
  Branches        ?        0           
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  Hits            ?      805           
  Misses          ?       99           
  Partials        ?        0           

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