don't forget to call (super-new) in db-stochastic-derivative-ctx%
provide density in prob/viz
pass #:cond special conditions to hmc-sampler
Using entry-value-map should also update the ll.
hmc-leapfrog-proposal must call the model on each small step.

Because we have to update the dist parameters in order to compute
dist-Denergy correctly.
bugfix: return a value when sampling from a pinned entry.
hmc-sampler spcond support works now.

The important bit is to leave pinned entries out of delta-x in the
leapfrog small steps so that they get reused.