Create and upload external skymaps

After discussions with Aaron Tohuvavohu, we decided it best for gwcelery to create Swift sky maps using the RA and dec information from Swift notices. Since Swift detections are so well localized, we can approximate the sky map as a single pixel (here an nside is chosen using an error radius of 1 degree, which is very generous considering Swift error is usually around 3 arcsecs).

The design is as follows:

1. Create sky map array using RA and dec info
2. Write this array to a sky map file
3. Upload this file and it's mollweide projection (.png) to the external event

To do:

• Create sky map
• Upload sky map
• Demonstrate this works: https://gracedb-playground.ligo.org/events/E187966/view/
• Create unittests