Add code to skip over events without a Physics trigger by default

[sdobbs]

Back in Aug. 2020, there was a modification to the analysis library to write out MC events that do not pass any physics trigger. The intention was to help with performing trigger efficiency studies, but this was not fully followed up.

This seems to only affect a small percentage of events, but it should be properly handled in the trees that are currently under analysis.

One way to go about this would be to ask people to change their DSelector to explicitly reject such events, e.g. skipping events with Get_L1TriggerBits() == 0

Another way would be to modify the DSelector to automatically skip such events. I think this PR does the latter, by making it so that the usual loops are not performed, by zeroing out the relevant array lengths that are used.

Whether this is the direction we want to go is something to be discussed...

[mashephe]

Giving this PR a bump since it shows up in some discussion with Will. Seems like a good solution would be to default to most correct (don't write out events that don't satisfy the trigger), but allow an option to override for study.

[jrstevenjlab]

This also came up in a discussion with @jonzarling on the eta cross section. I like making this the default and reminding people to do this test with and without the trigger requirement to see the impact on their cross section studies.