- the MCMC sampler draws from the square root of the PDF (1/2 the log PDF)
  to improve convergence in the tails.  this patch moves the divide-by-two
  out of the C extension code into the Python calling code for clarity.
  the C extension code exports a symbol "posterior" but because of the
  divide-by-two it wasn't really the posterior, now it is.  doing the
  division in Python slows the code down a bit but not significantly.