Extract reference time from data collection

[bmorris3]

The reference viewer x-axis is set by TimeScatterView._set_plot_x_axes, which currently requires that you pass in a LightCurve with a reference time in the meta attribute. The function already requires you to pass a DataCollection, which should also have at least one entry with a reference time on its TimeCoordinate.

This PR allows you to set the x-axis without a LightCurve, which is useful if you want to change the axes for an already-loaded light curve in the data collection.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.08%. Comparing base (7b78df5) to head (2bd9c6e).
Report is 55 commits behind head on main.

Files with missing lines	Patch %	Lines
lcviz/viewers.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #170      +/-   ##
==========================================
- Coverage   93.92%   90.08%   -3.85%     
==========================================
  Files          37       41       +4     
  Lines        1598     2188     +590     
==========================================
+ Hits         1501     1971     +470     
- Misses         97      217     +120     

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