(cleanup) add missing Copyright; delete empty file.
hmc: move hmc-step and hmc-acceptance-threshold to separate files.
hmc: record answer after last db propagation.

Saves us one extra re-evaluation of the model.
Get rid of hmc-sampler%.  HMC is now an mh-transition<%>.

Update example to demonstrate that (send info model) now works with HMC.
Update HMC example and scribblings.