Dropout PV system ID & PV metadata

[JackKelly]

During training:

With some probability (30%?), zero-out the output from the PV system ID encoding. And, separately, zero-out different elements of the PV metadata.

Then, during inference, just feed the network zeros for any missing metadata, or zero-out the ID encoding if we haven't seen this PV system during training.

With luck, this should clearly fatten-out the probability distribution of the predicted PV yield.