Plot ground truth OSI-SAF if forecast date has ground truth available

[tom-andersson]

In the absence of live validation metrics for hindcasts, it would be really useful to visualise the true sea ice state at the time of the forecast date. Then one could easily produce comparison figures between what was predicted and what was observed.

Ideally, this panel would show 'no data' or similar for dates from the future or dates with missing OSI-SAF data. Simpler would just be to not add the plot if no data is available (ie it would just disappear).

We could also have a separate slider for the OSI-SAF date for more control (ie so it isn't just the forecast date). Although this would be a bit more involved to align the forecast date with the ground truth date.

This relates to #7 around updating the dashboard to support hindcast analysis.

[JimCircadian]

I'm not inclined to do this, as it has a considerable impact on the functionality of this tool in that we have to start downloading and processing data. Perhaps a feature down the line when the data store is federated more effectively across the various tenants, but we're a ways off that. Closing this as invalid as conceptually it's nice, but in reality it's not pragmatic until it's called for by a user requirement, at which point it requires much wider consideration far outside the scope of this tool.

[JimCircadian]

Further reflection on where we do this has made me keen to keep this open, but it might not be in this particular component that we provide this functionality 😉