Variable number of parameter MCMC plots

This change allows the user to change the number of parameters for which the MCMC convergence diagnostics are shown. The user needs to set a value for options_.nparMCMCdisp for an aribitrary number of parameters to be shown, but defaulting to 3 if no input is supplied. This change also defaults to 3 if the user for some reason enters the value 0, as otherwise the code gets stuck in an infinite loop that keeps on opening new figure objects.