Increase speed of MCMC methods

[bayesianbrad]

We need to make dramatic increases in speed. So we need to take advantage of any incremental increases.

Potential easy increases

• [] Generating momentum. For continuous parameters we should be able to do this in one swoop and then distribute the momentum accordingly. For discrete it should be a very similar process
• [] Add multi-core support for running multiple chains
• [] Find more ways to use computation of log_pdf rather than unnecessarily making more calculations.