Implement stacked point-source analysis with public data

[martwo]

With the latest SkyLLH code, it is possible to implement a stacked point-source analysis using the public data. However, a few components of the current PS analysis with the public data might have to be updated to support several sources.

I recommend to add the following file for the stacked public data PS analysis:
skyllh/analyses/i3/publicdata_ps/time_integrated_stacked_ps.py

Looking through the public data classes, the following classes are already stacking ready:
skyllh.analyses.i3.publicdata_ps.backgroundpdf.PDDataBackgroundI3EnergyPDF
skyllh.analyses.i3.publicdata_ps.pdfratio.PDSigSetOverBkgPDFRatio
skyllh.analyses.i3.publicdata_ps.detsigyield.PDSingleParamFluxPointLikeSourceI3DetSigYieldBuilder
skyllh.analyses.i3.publicdata_ps.signal_generator.PDDatasetSignalGenerator

The following classes still need adjustments for multiple sources:
skyllh.analyses.i3.publicdata_ps.signalpdf.PDSignalEnergyPDF
The get_pd method currently takes evaluates all events. It should take events only from a given source index.
skyllh.analyses.i3.publicdata_ps.signalpdf.PDSignalEnergyPDFSet
This class is currently instantiated for a particular single source declination. It should probably take a SHG are constructor argument.