hmc.run: Run one HMC chain
In jtimonen/mc2: Markov Chain Monte Carlo Samplers

Run one HMC chain

1	hmc.run(log_prob, hamiltonian, x0, iter, step_size)

`log_prob`	a function that takes input `theta` and returns a value that is proportional to the log probability density at `theta`
`hamiltonian`	a function that takes as input the location and momentum and returns their time derivatives
`x0`	initial value
`iter`	number of iterations
`step_size`	leapfrog step size