Add HMC Swindles

[rlouf]

There are several algorithms in https://arxiv.org/abs/2001.05033 that should be simple to implement.:

• HMC-COUPLED is a kernel that runs two identical kernels with the same rng_key;

def hmc_coupled(rng_key, states):
    states, infos = jax.vmap(kernel, in_axis=(None, 0))(rng_key, states)
    return states, infos

• HMC-ANTITHETIC is a kernel that runs two kernels with opposite momenta and identical rng_key.

[rlouf]

Seemingly unrelated, but Metropolis-within-Gibbs has been shown to work well (https://github.com/blackjax-devs/blackjax/discussions/275). So this is definitely feasible.