Implement effective sky fraction functions

[zonca]

Implement functionality in mapsims to compute effective sky fractions for signal and noise dominated maps as defined below.

I would actually say that a more "standard" definition would be either fsky=< Nhits^2 >/< Nhits >^2 or fsky=< Nhits^2?>^2/ < Nhits >^4, which correspond to the effective number of modes for noise- and signal-dominated maps.

Originally posted by @damonge in #5

[zonca]

those are general enough, maybe add them to healpy instead