Add PMT Beam Signal timing to ICARUS CAFs

[aheggest]

Description

This pull request reads the special PMT Beam timing signal (RWM) into the CAFMaker which enables us to reconstruct the Beam Bunch structure seen by ICARUS PMTs.

The PMT Beam signals were originally stored in an icaruscode data product icarus::timing::PMTBeamSignal, but icaruscode PR #832 and sbnobj PR #130 together move the data product to a SBN common area, sbnobj/Common.

A std::vector<sbn::timing::PMTBeamSignal> is read into the CAFMaker and then passed to FillICARUSOpFlash if the PMTBeamSignal is valid. If running on MC, offbeam data or stage1 files without the sbn::timing::PMTBeamSignal data product, the srflash.rwmtime variable will be left to it's default value kSignalingNaN in SROpFlash.h.

Within FillICARUSOpFlash, two new helper functions defined in sbnobj/Common/PMT/Data/PMTBeamSignal.cxx are called.

• SelectFirstOpHitByTime reads in a single OpHit and returns a startmap and risemap (holding the start and rise time of each OpHit channel).
• getFlashBunchTime reads in the risemap and std::vector<sbn::timing::PMTBeamSignal> RWMTimes, finds the mean between the first OpHit RiseTimes on opposite walls to get the Flash Interaction time wrt the RWM time.

This PR is a companion to sbnanaobj PR #141. This PR requires icaruscode PR #832 and sbnobj PR #130 also being merged.

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[mvicenzi]

Generally looks good. Main comment is: we need to make sure this doesn't break if the product is missing (MC files, older data files). Have you tested this as well?

[mvicenzi]

My comments were addressed by the recent changes, so I approve.