<tt>MCMCCalculator</tt> is a concrete implementation of the
<tt>IntervalCalculator</tt> interface.  To use it one needs to specify the <tt>ProposalFunction</tt>.
There is a base class for ProposalFunctions and one concrete implementation: <tt>UniformProposal</tt>.
Support for other proposal functions will be added in the next release.
The <tt>MCMCCalculator</tt> scans the space of the parameters of interest and nuisance parameters and
produces a Bayesian posterior.  In this version, the prior must be added 