Convert nowcast callback exporter to xarray

[mats-knmi]

Since xarray does not natively support incremental writes (https://stackoverflow.com/questions/66231575/xarray-appending-or-rewriting-a-existing-nc-file), we need to find a solution for the callback method.

• One option could be to create the initial dataset using dataset.to_netcdf(...) and then incremental write to it using the netcdf4 library directly.
• Another option is to use dask, as that is supposed to make it possible to stream data, but I don't know how that works and if it even fits our usecase (a dask expert, might be able to pick this up).
• Final option is to just create separate files per timestep