Change Python bindings method for overriding virtual methods of C++ classes

To override the C++ virtual method 'DoStart', we new define a method in Python called 'DoStart', whilst before we would define '_DoStart'.  This change is to align pybindgen generated bindings with the other python bindings tools out there, such as boost.python, swig, and sip.  It is also simpler to understand for users.

bindings/python/ns3modulegen.py

@@ -12,7 +12,9 @@ sys.path.insert(0, sys.argv[2])
 
 from pybindgen import FileCodeSink, write_preamble
 from pybindgen.module import MultiSectionFactory
+
 import pybindgen.settings
+pybindgen.settings.deprecated_virtuals = False
 
 from ns3modulegen_generated import module_init, register_types, register_methods, register_functions
 import ns3modulegen_core_customizations