Have started a fairly major revision of the MCMC parts of the code.
The immediate aim is to generalise the definition of the MetropolisHastings
class to better allow for user-defined proposal distributions. Specifically,
I want to be able to use the chain sample covariance for a multivariate
normal proposal distribution. So far I've restructured the code so that
proposal distributions are defined as separate classes in their own right,
with methods such as pre_tune and step.