Update to ultra-legacy

[kpedro88]

This PR updates the configurations to ultra-legacy using CMSSW_10_6_X and adds several useful features and improvements:

• Propagate modifications to corresponding version of genproductions repository for UL (https://github.com/cms-svj/genproductions/tree/Run2_UL)
• Use of concurrent generator modules*
• Improve pileup dataset pickling script
• Multiple options for HLT simulation: use legacy releases (as in official production) or simplified relval menu (faster and simpler, if realistic HLT output is not needed)
• Update tunes and PDFs
• Incorporate GenJet pT filter for boosted signals
• Incorporate official CMSSW UserHook for SUEPs (Backport of Factory for adding UserHooks to PythiaHadronizer and SUEP decay pythia Hook cms-sw/cmssw#36509)
• Add runProd.py script to create and submit entire production chains in one click
• Reorganize and update readme

* It should be noted that the concurrent Pythia generator module currently does not work with RandomizedParameters when LHE arguments are included in the parameter sets.

The basic examples have been tested, but not exhaustively. Please report any errors.

When this PR is merged, the Run2_UL branch will become the default branch. The pre-legacy setup will be retained in the Run2 branch.

attn: @Keane-Tan @cmadrid1 @tklijnsma @snabili @chrispap95 @kdipetri