Add a convenience function to stitch multiple quarters straight from the archive

[barentsen]

I wonder if we can have kepstitch take the name of a planet or KIC number, and have to grab all the quarters for that target from MAST and stitch it all together, so a user can obtain a full-mission lightcurve for a target by means of one simple command.

[evh3rd]

I think that the class we discussed to retrieve from the archive could support this. My plan is to create a "kepget" that will call a class to retrieve the light curves according to KIC, KOI, or planet and a list of quarters. The class would retrieve and cache the light curves (as in kplr). It would return a list of light curve file names (in the cache) that could be passed to existing kep functions as is. I would allow like to support --kepler, --k2, --tess, ... to determine which mission is the target of the retrieval. For --kepler, a list of quarters would be provided (default to all?). For K2, possibly campaign specifiers, for TESS, I don't know yet. ;-)

[evh3rd]

See issue #147. I was ill over the Christmas vacation, so I have not had the energy to work on it. Just getting back to it now.