Enable phase_center inputs to be SkyCoord

[DanRyanIrish]

This PR enables the phase_center inputs to the Visibilities calculating functions to be given as a SkyCoord. The result is that the Visibilities object knows its phase center in a valid coordinate frame, and hence enable imaging routines to return Maps in the same coordinate frame.

[codecov]

Codecov Report

Attention: Patch coverage is 90.32258% with 9 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@ea833d9). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
xrayvision/visibility.py	50.00%	7 Missing ⚠️
xrayvision/coordinates/frames.py	95.91%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #77   +/-   ##
=======================================
  Coverage        ?   89.37%           
=======================================
  Files           ?        8           
  Lines           ?      969           
  Branches        ?        0           
=======================================
  Hits            ?      866           
  Misses          ?      103           
  Partials        ?        0           

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[DanRyanIrish]

Do not merge until #76 is merged.