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