Use multiple different engines in VMC

This is an extension allowing the VMC package to run a simulation
with multiple different engines at a time. Tracks can be transferred
among engines during a simulation run based on conditions specified by
the user.

Important notes on the extensions:
  1) This extension preserves backward-compatibility in the sense that
     user code relying on the former version of VMC is still running
     with the extended version. Was tested with GEANT3_VMC@v2-6 and
     GEANT4_VMC@v3-6-p1.
  2) A shared simulation is only possible when TGeo is used for geometry
     construction and navigation.
  3) A TMCManager singleton object is responsible for handling multiple
     engines and can be obtained on request calling
     TVirtualMCApplication::RequestManager() during construction of the
     user application class.
  4) The introduced TMCParticleStatus objects hold additional
     information to keep track of properties when a track is transferred
     between engines.
  5) When a track is interrupted in one engine to be transferred to
     another, the geometry state is cached in the form of a
     TGeoBranchArray object. It will be used to initialize the navigator
     when this track is picked up for further transport in the next
     engine. This is especially useful/required when a track is
     transferred at a volume boundary in order to be picked up in the
     entered volume and not in the one just left. This is a main reason
     why geometry management is forced to be done via TGeo.

A more comprehensive introduction concerning the usage and
implementation in the user code can be found in the
montecarlo/vmc/README.md

Further note:
  This commit also applies the clang format to the modified and new
  files.