Issue title: astropy.timeseries.TimeSeries should be returned when sensible
 Issue body: The idea came up on our (UW) time domain meeting that it would be great to receive `astropy.timeseries.TimeSeries` from astroquery queries where the object is actually a timeseries. 

E.g mast could support this alreay, and in the future it would be a good way to provide ZTF/LSST etc timeseries to users.
 Comments: 
 Comment 0: We should add labels for the affected modules, but atm I'm not 100% which could be do this right away.

cc @ceb8 

 Comment 1: I think this is a great idea! mast is not set up to do this right away (unless TimeSeries can handle target pixel file data, I didn't see anything about that in the docs, but I didn't look that hard), but it would be a nice thing to do in conjunction with the work suggested in this issue: https://github.com/astropy/astroquery/issues/1374
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