Add --dt option to TimeSeries plot different times

Created by: areeda

This is in response to a long standinf requests to be able compare time series of the same channel at different times. An example is https://ldvw-dev.ligo.caltech.edu/ldvw/view?act=getImg&imgId=152709

It uses a switch on the command line (--dt) which forces all time series object to be plotted as though they have the same start time, with the legends extended to show the actual start time.