Add option in the HltEfficiencyChecker yaml config to turn on DEBUG mode

Just provide debug_mode: True in your yaml under the job options.

Also protect against a potential ZeroDivisionError that could occur in hlt_line_efficiencies.py if running with a very low number of events.

Requires Moore!849 (merged)

TODO:

• Add to documentation that this option is there -> Moore!849 (merged)