Add flag for when pv generation is NaN

Currently in the sites pipeline, when there is no generation data for sites, we infill any NaNs with 0's.

This as @JackKelly has stated is quite "mean" to the model, expecting it know when we don't have site generation data but are passing it 0 values.

One solution to this could be to change the value from 0, to a very specific number that the model could learn that when site generation is that value that there is no real generation.

Another solution which I propose in this issue (thank you @Sukh-P) for the idea, is to pass a flag to the model for each site generation for when it is available. This could be an extra datavariable which is 1, for when site generaiton exists and 0 for when we have replaced NaNs in the dataset for 0. This hopefully allow the model to learn when there is No PV generation better.

Open to thoughts on other solutions and how this could be implemented.

Start by locating the sites pipeline and the step that replaces missing generation values with zero. Review how site-generation variables are passed to the model, then determine where the availability flag should be added and how it should behave for observed versus replaced values. Done means the pipeline supplies the flag consistently and coverage is validated with relevant tests or sample data.