Maybe provide surface height and land/sea mask as coordinates on all batches?

[JackKelly]

In power_perceiver, I'm planning to encode the surface height (and maybe land/sea mask) as part of the position encoding.

At the moment, it's actually quite tricky to align the topo data with the satellite data (because xarray.align doesn't like the fact that the satellite data's OSGB coords are each 2D. So we have to reproject the topo data first, and then align it).

It'd be much easier if the topo data was just another coord in the satellite data and the PV data.