Source.plot_lightcurve(): Add option for x-axis to use Delta T rather than datetime

[ddobie]

For targeted searches (e.g. GWs, GRBs, SNe) it makes sense to think about the temporal evolution in terms of "days post-event".

I propose adding an option to plot lightcurves in these terms, e.g. source.Source.plot_lightcurve(start_date='2018-08-14') which will relabel the x-axis in terms of days since start_date.