Significant MC Workflow restructuring (#1790)

* Significant MC Workflow restructuring

This is bigger development related to https://its.cern.ch/jira/browse/O2-3622

It makes it possible to run embedding workflows with the --pregenCollContext feature.
This means that the collision structure of all timeframes are now systematically generated
before any event generation or GEANT transport stage.

The O2DPG MC workflows are restructured so that they

* generate the collision structure (collision context) at a very high level
  - this contains information about how collisions are composed (background-signal)
  - this also fixes the vertex of the collisions
  - this fixes the number of background and signal events to be generated
  - this is now done for all timeframes at once instead of in isolation
    (necessary to know how many background events are needed)

* thereafter we generate and transport background and signal events

This restructuring leads to more consistent workflow construction
and smaller code. The old option --pregenCollContext is now the enforced
default.