Add ability to perform Hamiltonian Monte Carlo

[adolgert]

Hamiltonian Monte Carlo samples a probability space of all possible trajectories, such that the trajectories are weighted according to their likelihood. This technique doesn't use a sampler to decide the set of possible trajectories, but it could use the samplers in CompetingClocks to calculate the likelihood of the chosen trajectories.

You set up a simulation with a modest number of events in each trajectory. You decide some observations of events. Then sample the space of trajectories consistent with those events. It sounds much harder than it is. It's a very good technique for this library.