Add option to have initial steps taken at a higher temperature.
Added mechanism for rejecting parameters in getLogLike

* getLogLike can now signal invalid params using a second argument.
  This automatically rejects the step.  The idea is that sometimes
  it's hard to know a parameter set is invalid without some complicated
  piece of computation only available to the likelihood computation.
* Make sure at least some steps are accepted during initial step set
  or burn in as a guard against some mistake causing all steps to
  be rejected by the above.
* This does mean it is important for the initial state to have
  actual likelihoods available.  If not, it's possible the first
  step will be rejected as invalid, and then the initial step to
  be propogated along in a way that skipFirstStep can't handle.
Add helper for computing log-likelihoods of most recent steps.

The idea is that it may be convenient, for expensive likelihood
calculations, to initialize the params without setting the likelihoods,
but then it's nice to be able to actually fill them in.

* Added ability to replace most recent step (replaceLastStep) in
  affineStepChunk and affineChainSet.
* Added calcLastLikeLikelihood to affineEnsemble to compute
  the most recent step likelihood in parallel.
* sample and doSteps now call calcLastLikelihood at the beginning
  to make sure they have valid likelihoods.
* A consequence of the above is that the initial position must be
  valid or the code throws an exception and crashes.
* Add test of replaceLastStep to chain_test
* Bump version number to 0.2.4.
Rename configure.in to configure.ac