Ensemble Land Use Change predictor models

[ofrancon]

A few predictor models are already available for the Land Use Change (LUC) demo. Create a new predictor by ensembling some of them. Ideally the ensembling method should be generic enough that it can be used with any combination of models.
It should then be possible to compare the ensemble with the individual predictors by using the 'robojudge' (see #49 )