Pynomad

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- made finding the nomad database dir more robust & able to find on other volumes (if not buried)
- removed SkyCoord functionality because performance was unacceptable (adding a SkyCoord object column to pandas DataFrame was incredibly slow no matter when/how the column was created, even at initial Dataframe creation)

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- add add_SkyCoord_field param to queries so that can toggle off if don't need. The creation of SkyCoord objects is relatively slow. (Biggest issue is that adding a column to a Pandas DataFrame is quite slow.)
- tried to change how the pandas DataFrame is created to address speed issues when SkyCoord column is included, but failed and stripped out the SkyCoord code
- made finding the nomad database dir more robust & able to find on other volumes (if not buried)
- fixed a few bugs with paths if _nomad_dir isn't given a trailing '/'.

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- include proper motion values in output dataframes (useful for making starlists for some observatories where they will calculate the proper motion for the specific date)

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- huge speed-up by moving where 'radec' SkyCoord field is added (now doing after winnowing of stars)

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- output DataFrame now has 'radec' field that is of type astropy.coordinates.SkyCoord
- added astropy>=1.0.0 dependency

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- bumped up pandas version requirement because of usage of df.loc[]

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