Add support for reading McEventCollection_p6 to 21.0-mc16d

Part of the work for ATLINFR-4725.

NB Any HepMC3 attributes including filterWeight will be lost when reading in McEventCollection_p6.

Build area was cleaned as per request posted in the DB. The full software build will be performed

This merge request affects 2 packages:

• Generators/GeneratorObjectsAthenaPool
• Generators/GeneratorObjectsTPCnv

Affected files list will not be printed in this case

added 21.0-mc16d Generators review-pending-level-1 labels

The system determined that CI tests (with names matching "^CITest_Trigger.*$") are not needed for this code change. They are not run. This is not an indicator to restart the job.

CI Result FAILURE (hash f1cb4229)

	Athena
externals
cmake
make
tests

Full details available on this CI monitor view. Check the JIRA CI status board for known problems
NEW: project list is tailored to contain projects affected by code changes
Athena: number of compilation errors 0, warnings 186
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 60833]

There's a few CI issues coming up here, could you please check whether any are related to your changes?
Albert (L1)

added review-user-action-required label and removed review-pending-level-1 label

removed review-user-action-required label

added review-pending-level-1 label

resolved all threads

added sweep:ignore label

Thanks for your responses, I think I can proceed with approving this then
Albert (L1)

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit 0f0431da

mentioned in commit jchapman/athena@aea156a1

mentioned in commit jchapman/athena@610ffe7a