Enabling onsource vs dq vetoes check in PyGRB

[pannarale]

In the PyGRB coherent search, the onsource and offsource trials must have the same lengths. This PR ensures that no vetoed time is included in the onsource segment, to avoid having onsource and offsource trials differing in length. If the onsource contains vetoed times, the workflow generator exits with an informative message.

Standard information about the request

This is a: new feature.

This change: has appropriate unit tests, follows style guidelines (See e.g. PEP8), has been proposed using the contribution guidelines

Links to any issues or associated PRs

See issue #4981

Testing performed

I took the GRB 170817A configuration file, and the workflow generation is completed. I then artificially moved the trigger time to a time with dq issues in one of the IFOs, and the workflow generation process exits in an informative way.

Additional notes

• The author of this pull request confirms they will adhere to the code of conduct