s/harmonic/hamiltonian/
Move HMC utilities to prob/private/hmc/
hmc: (label L (observe-at ...)) and (derivative (observe-at ...) ...) work.
Merge branch 'akliger/to-upstream' into wip/hmc-improve
hmc: If leapfrog moves into a region with likelihood 0, reject the entire leapfrog big step.
Linear regression using Hamiltonian Monte Carlo.