plot.mortalityDataWithFits: method to plot a set of mortalityFit objects and the data...
In dfeehan/mortfit: Fit and evaluate mortality models

@param x mortalityFit object @param y not used @param Dx if TRUE, plot in terms of deaths; otherwise, use central death rates @param singleplot if TRUE, plot all of the fit objects on the same plot; otherwise, return a list of plots @param ncol if not NULL, the number of columns in the array of plots produced @param pdffile the name of a .pdf file to save to (if NULL, plot is displayed to the screen) @param ... other args, which are passed on to the pdf device @export