Added inverse covariance function
First attempt at adaptive hessian guided MCMC

* Use the sigma parameter to scale the size of the jumpbs during
  hessian phase, not just during the annealing phase
* Adjust sigma up and down by multiplying or dividing by a scaling
  factor, rather than just be incrementing or decrementing
* Decide whether to adjust sigma based on the "local" acceptance
  rate, i.e., the number of accepts within a recent window of steps.