Add a way to make 2001:001 == 2001:001:00:00:00, and other conveniences

[taldcroft]

Having 2001:001 go to 2001:001:12:00:00 is useful in some contexts, but often annoying. Perhaps add a new Time class that changes the API a bit and makes some assumptions for convenience. Other examples:

Float/int inputs 1900 < input < 2200 are assumed to be frac_year.
Float/int inputs 1900001 < input < 2200000 assumed to be greta.

Probably use this new class for fetch, xija.