hmc: use last trace db for initial sample, if available.

Run the gradient+initial sample mode if either there's no gradient or
there's no initial sample.
gaussians circle: lower the stddev; use mh for burn-in, then hmc.
hmc: zone support.
hmc: Only count entries that match active zone in db-Denergy.
Ex: hybrid model multi-zone mixed HMC transition.

Model has discrete and continuous random variables.
We use HMC on the continuous non-structural choices interspersed with
multi-site MH over the whole model.