Create a proportion of examples without PV data, outside the UK #93
Description
We currently only have PV data for the UK. We will at some point want to get PV data for elsewhere but, in the meantime, we'll need nowcasting_dataset to optionally output examples from outside the UK (to train the "image prediction" part of the model on the entire geospatial extent of the satellite imagery).
Maybe we should create two sets of batches on disk: one set which always has PV data (and is over the UK), and another set which is always from outside the UK (and doesn't have PV). Then the ML training script can mix-and-match examples on the fly to vary the ML training curriculum. To keep each batch ballanced, the ML training script will need to load at least two batches at once from disk (one with PV data, the other without) and create a single batch with a mixture of examples.