Various fixes to EventManagers

[titodalcanton]

This addresses, at least in part, #4416 and #4632.

It only affects the HDF5 output of pycbc_multi_inspiral, specifically:

• The single-detector snr_* datasets are renamed to just snr. This will break code downstream of pycbc_multi_inspiral, which will have to be modified (probably in a later PR though).
• Per-detector gating information is now stored correctly.
• The template_duration dataset is removed, the idea being that template parameters should always be taken from the bank file instead. Note that a few plotting codes in the all-sky offline search workflow do need template_duration to be in the trigger files. I think the main reason for this is that not every bank generation code adds the template duration dataset when generating the bank, so historically this has been done in pycbc_inspiral instead. I added a comment to explain this next to the relevant code in EventManager. I think this is not ideal, so we should try to avoid this in PyGRB workflows.

I have broken down the changes into small and logically-meaningful commits, hopefully this makes review easier and faster.

[titodalcanton]

It seems a few things in the all-sky coinc offline workflow actually depend on template_duration being in the output of pycbc_inspiral (I count at least plot_singles_timefreq, page_foreground, page_snglinfo, page_coincinfo). So I will revert that change in order to keep this PR focused on multi_inspiral.

[jakeb245]

Looks good to me. It will break things downstream in the PyGRB workflow, but it should be easy to fix and will simplify the code.