Allow customization of the BasePackage and Model Directory for the GenModel

[CamilleLetavernier]

The default genmodel is sufficient to generate some java code, but in most cases, the user wants to at least customize the following fields:

• base package (Otherwise, the generated packages match the Ecore package name, which can't include segments)
• target project / source folder

These values should be additional, optional parameters in the GenModel creation command