add mixture model over constellations

• write a special case of RADecDistribution (called Constellation) that is a top-hat for a given polygonal bounding box.
  • make sure this has an attribute which returns the area of the top-hat domain (in stradians? sqr deg?)
• write an auto-declaration loop that iterates through constellations and declares a class for each (a subclass of Constellation)
• write a mixture model over all Constellation classes. Parameters are the probabilities of that constellation (note that larger constellations should have larger probabilities).
  • make the default behavior to compute mixing fractions so that the probability per sqr degree is uniform over the whole sky