Add the ability to generate random Monte Carlo samples from a HealPixLocalization

[AdamGoldstein-USRA]

Since a HealPixLocalization object represents a localization posterior on the sky, we can generate Monte Carlo samples from that posterior. Propose adding an object method HealPixLocalization.sample(size=1) that will return Monte Carlo samples from the localization posterior. It would called in the following ways:

# generate a sample
one_sample = hpx.sample()

# generate 10 samples
samples = hpx.sample(size=10)