Add an ensemble model to the Colab notebook.

[JayThibs]

Create an ensemble of the segmentation models. Use the max value of all the models as the ensemble value. This will bias the model towards saying it is floodwater rather than missing out on floodwater. This is great because in production we'd be less like to miss out on floodwater events (which is much worse than underpredicting them), and it also has been shown in other projects that the models will typically miss out on floodwater more often than not so this can correct for that.