add sample method for maps

[esheldon]

e.g.

smap = HealSparsMap(...)
ra, dec = smap.sample(size=1_000_000)

This will allow different map types to generate randoms differently but in a transparent way to the users. E.g. weight maps would gen randoms proportional to the value in the map. Integer maps may want to generate them uniformly, or only when certain bits are not set, etc.