provide method to return the root model object while creating a new file

[girishchandranc]

Currently, autosarfactory requires user to pass a default arpckage name while creating a new file and the api returns the created arpackage object. There is no provision for the user to add another package in the root level. And hence, there is a need for a new method to just create the file and return the root model object and allow user to create any number of packages under the root node