Beam time structure simulation using SBN (shared) GENIE configuration for BNB and NuMI

[PetrilloAtWork]

This is a revamp of the GENIE generation configuration jobs with BNB and NuMI beams:

• it now relies to a shared SBN configuration (meaning that the new shared configuration is kept in sbncode and it is available to SBND too — it's up to them to use it or not);
• it enables the beam timing structure simulation.

Some checks on the beam timing can be found in SBN DocDB 39065.

Reviewers:

• @leoaliaga, generator expert
• @betan009, NuMI analyses coordinator
• @woodtp, expert on NuMI and its flux simulation

Dependent on SBNSoftware/sbncode#492.

[woodtp]

Looks good to me--approving.

Tangentially, I've opened an issue (#789) to remind myself to update the flux files to the newer set @PetrilloAtWork has made available for us in the sbn stash.