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.

The function getAttrOrDefault(xml::Element e, xml::XmlChar attr_name, T default_value)
will return the attribute converted to to type T but if it is not found it will return
default_value. When building new detectors supplying this is useful for
supplying default attribute values.

Improve the CMS example. Add word for the raw generator status of generator particles to Geant4Particle

Improve the CMS example. Add word for the raw generator status of generator particles to Geant4Particle

Improve the CMS example. Add word for the raw generator status of generator particles to Geant4Particle