Add TimeGPS class to Time (=seconds since 1980-01-01 00:00:00 UTC = TAI -19 s

@taldcroft - I was asked by David Kaplan whether it would be possible to add a gps format for Time, since this is used a lot at MWA, and seems generally useful. The pull request implements the `TimeGPS` class following the `TimeCxcSec` example (aside: isn't that a bit Chandra-centric? What about poor XMM? ;-), and adds tests and documentation.