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
Implement background computation step