Add user defined framework context to DDG4and make the G4RunManager instantiation a plugin this should consequently allow for user defined specializations.

Add user defined framework context to DDG4and make the G4RunManager instantiation a plugin this should consequently allow for user defined specializations.

Add user defined framework context to DDG4and make the G4RunManager instantiation a plugin this should consequently allow for user defined specializations.

Added createGearForSiD plugin code

Added createGearForSiD plugin

Since gSystem.load() can return 1 if the library has already been
loaded, there are cases where requiring `result == 0` raises a false
exception.  This commit changes the required condition to be `result >=
0`

Since Python3 moved to wide character in argument of PySys_SetArgv(), DD4hep is
currently incompatible with Python3.  For now, let's resolve this by requiring
Python2 via CMake.