HealPixPrior (Resloves Issue #419)

Hi all, This MR introduces a HealPixMapPriorDist object to be used to create a 2D or 3D joint prior in ra/dec (ra/dec/distance) according to a passed in HealPix map or .fits file to address and resolve issue #419 (closed).

This PriorObject inherits from BaseJointPriorDist and needs to overwrite three methods (_rescale(), _sample(), and _lnprob()) To illustrate this is working I sample 5000 samples from the prior and show below that these samples are distribvuted according to whats in the HealPix Map .fits file.

Here I show that the _rescale() method is also working by generating 5000 samples uniformly on [0,1] then rescale to the HealPixPrior which also matches with what is in the map file.