Resolve circular problem with zerolag ranking stat PDF when starting online analysis from scratch

The issue: The gstlal_ll_inspiral_trigger_counter expects a marginalized zerolag ranking stat PDF to operate on, but this is initially bootstrapped by taking a dist stat PDF from a single bin.

The problem is that these files don't get created until the inspiral jobs start up from the first time. That's fine, except you have to create the file after the fact and can be error-prone.

The solution proposed in this merge request: Allow passing a ranking stat file via --bootstrap which allows this trigger counter to generate an empty zerolag dist stat PDF file if file doesn't already exist. This same machinery is used by the gstlal_inspiral jobs which works very similarly.

This option is also passed in when creating the online analysis DAG. Note that a reference file from any of the bins will do.