Using geocenter time in PyGRB timeseries plots of all timeslides

[pannarale]

The executable that plots SNR timeseries of PyGRB analyses currently overlays slides using the detector time, when showing single IFO timeseries. This implies that:

1. the onsource of a closed box is not correctly omitted for slid detector data, because geocenter start and end times are used to identify the onsource window;
2. the horizontal axis values are mixing different meanings for different slides.

When the user wants to look at more than one slide at once, the correct thing to do is to use geocenter time. In other words, detector time is used only when a single slide is plotted, for a single detector.

With this switch, this PR address both issues listed above.

This is a: bug fix (item 1), improvement in results display (item 2)

This change affects: PyGRB

This change changes: result plotting

Testing performed

Here is a plot of all time slides for a specific GRB at L1

Here is how it looks with these code changes

And here is a plot of a single time slide after the code changes, to ensure that the functionality is preserved

Additional notes

In the new version, the sliding is manifest (peaks are smeared rather than overlayed)

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