Add a conveniences method for PhotometryData

[mwcraig]

Adding a to_lightcurve method that makes a LightCurve object from the lightkurve package would make PhotometryData much more useful. Those objects have methods for performing a periodogram (Lomb-Scargle or BLS), phase folding, plotting, etc. So lots of extra functionality without much effort on our part. A LightKurve object is a subclass of astropy's TimeSeries, so we get both for free.