Modify for 2019 trigger information.

[mholtrop]

The current implementation of HpsEvent.h with a separate "int" for the pair0_trigger, pair1_trigger etc, was a poor choice, as these values are booleans. Now we will have a lot more possible trigger bits. I will change the trigger bits to a 32-bit bitwise structure. Unfortunately, doing this directly will complicate the reading of old dst files, which will contain the old 'ints'. It may require a custom streamer for the HpsEvent class to make it work transparently.