Refactor Generator object to reduce its responsibilities

Right now the generator has to

1. parse the command line arguments
2. load the ecore model
3. modify the model
4. write and compile the model

This violates the single responsibility principle and should be refactored.

As a result, the generator should only take care of the actual workflow (items 2 to 4) and be invoked by another service which handled the command line arguments already and only needs to worry about setting up the generator accordingly.