Class for managing and sampling from sets of chains
Added convergence_criterion function to MCMCSet
Function to indicate whether chains have been pruned
Made convergence.py work with MCMCSet

And got rid of convergence_criterion method from MCMCSet. Figured that
this way convergence_criterion could be made a reporter. Though I could
always have written a reporter function...meh, this works OK
Modified report funcs to take MCMCSet instead of single chain
Rearranged code for reports and convergence tests

Added a reporter wrapper for convergence in convergence.py rather than
adding a reporter decorator to the function itself. Also added a
num_chains reporter.