add priority setting for condor jobs

Summary

This MR adds a priority arg that allows users to set a condor Job Priority for the bilby_pipe jobs.

From the HTCondor docs:

Job priorities allow a user to sort their own jobs to determine which are tried to be run first. A job priority can be any integer: larger values denote better priority. So, 0 is a better job priority than -3, and 6 is a better than 5. Note that job priorities are computed per user, so that whatever job priorities one user sets has no impact at all on any other user, in terms of how many jobs users can run or in what order.

Scope for further improvement

This feature could be expanded later on, to make analysis jobs to have higher priority than generation jobs (so analysis are completed before starting new generation and analysis jobs, etc).