Improve notify_changes job and add workflow to avoid duplicate pipelines

Changes introduced

• Do not run branch pipelines for branches that have open MRs - only run associated MR pipeline
• Check for produced EVNT files in notify_changes job
• If [skip modfiles] is used and modified files in pre-existing DSID directories are found the MR is always reassigned to the conveners
• If existing EVNT files are found for the DSIDs that contained modified files, the pipeline fails completely
• Undo change in !2118 (merged) since the change there meant that one would be allowed to change an existing filename and make it identical to another filename (which makes no sense since we do not allow identical filenames)

Tests

Existing DSID with produced EVNTs and [skip modfiles] => Works

https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/25126280

Pipeline fails and MR is reassigned

Existing DSID with no EVNTs produced and [skip modfiles] => Works

https://gitlab.cern.ch/atlas-physics/pmg/mcjoboptions/-/jobs/25133000

Pipeline is reassigned but there is only a warning given and in principle the MR can be merged

Issues resolved

Closes #169 (closed) #191 (closed)