Variable number of parameter MCMC convergence plots

This change allows the user to modify the number of parameters for which the MCMC convergence diagnostics are shown. The user needs to set a value for options_.nparMCMCdisp in the mod file before the estimation command for an arbitrary number of parameters to be shown. The code defaults to 3 if no input is supplied. The code 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 but never plotting anything.