Add summary to EffectMeasurePlot

[pzivich]

Like forest plots, there zEpid should calculate the summary measure of multiple studies into a single summary estimate (and add that estimate to the bottom of the plot).

This would be for a systematic review. Currently, I made the function to summarize stratified results or results across several models. This would be a further generalization.

I should also clean up this code while I am at it (I wrote it a long time ago and it can be improved). Also it would be ideal to remove t_adjuster and come up with some algorithm to optimally space the table (but leave the option for fine adjustments)