Excarta slicing not working correctly in Windnet

[jacobbieker]

Describe the bug

When building batches with Excarta data and WindNet, the output format is a 4D tensor, not the 5D one expected. The output from the load_excarta is the same as for other NWPs as far as I can tell, so not sure what is happening there at the moment.

Expected behavior

The output to be a 5D tensor of data, same as other NWPs, like ECMWF.

Additional context

Add any other context about the problem here.