Reload GenModel before generating code

[CamilleLetavernier]

(Note: this is an issue with the WiP codegen branch, that isn't integrated yet)

The GenModel file is created from the current version of the Ecore model, but isn't reloaded before triggering code generation. If the Ecore model has changed in the meantime, the code generation will fail.

We need to either provide a mechanism for the user to manually reload the Genmodel, or do it automatically before generating code.