add an option to train_model to create the project and stop

[alastair]

Sometimes you may want to create a project then edit the generated project file before actually doing the training. It would be good to add a flag --notrain or --create-only to just create the project and then stop before doing the training.
The next time you run train_model without this flag it should pick up the existing project and train the model as expected.

[dbogdanov]

Yes, this is useful.