Add a ModuleDefinition to the document

def add_module_definition(self, module_definition):
        """
        Add a ModuleDefinition to the document
        """
        if module_definition.identity not in self._module_definitions.keys():
            self._module_definitions[module_definition.identity] = module_definition
        else:
            raise ValueError("{} has already been defined".format(module_definition.identity))