Add button to auto-chop light curve into segments.

[scfleming]

gPhoton light curves often have large gaps between visits to a target, meaning the default plot at full x-range is basically unreadable. This ticket is to explore some way to allow the user to tell GGUI to split the full light curve into visits based on the gaps between observations and plot each of them with proper x-axis ranges.

I could imagine a button added to the light curve canvas that auto-chops or splits, whatever terminology we want to use, the light curve. The individual light curve segments could be plotted in different windows, or maybe a single new window with a "next" and "previous" button, somethin like that.

[scfleming]

This ticket is ammended to make it so that each chop segment is included as a Data Object, with added columns of relative start and end times so those could be plotted.

[duytnguyendtn]

Moved to milestone v0.5 to distinguish between AutoChop "On Demand" vs "Upon Load"