Create multipix subpackage
Use a class to encapsulate the gibbs sampler
Create a model data set for the gibbs demo
Setup class structure for MultiPixelGibbsBgModeller
ThreeParamLnProb encapsulates ln posterior prob

We want to make posterior prob functions a callable object so that we
can embed FSPS and data sets into each ThreeParamLnProb instance that
could live on separate machines.
Add notebook documenting the multipix Gibbs sampler
Re-align the PGM
Edits to the multipix notebook
phi and B parameters should be passed to each posterior sample
Small edits to multipix_daft notebook
Pre-allocate memory for separate theta,phi,B chains
Set up parallelized pixel sampler
Fully initialize the pixel lnpost in the parallel case
Create a lnpost for local use
Initialize walkers for pixel emcee sampler
Make an persist blobs via the pixel posterior

Since the posterior function/class knows what blobs it returns I have
the gibbs sampler class use methods *within the posterior class* to
prepare this memory and add blobs to the blob structured arrays.

Note there is a separate blob array for each pixel.
Create GlobalThreeParamLnProb

This is the posterior for distance.
Implement global-level sampler
Implement gibbs sampling step for background
Implement replication of blob chain