Switched to NetCDF4 storage of predictive output

Merged Chris Jewell requested to merge cherry-pick-63519eb3 into master

Changes:

  1. Where xarrays were pickled to disc, we now use xarray.to_netcdf
  2. Where xarrays were read from Python pickles, we now use xarray.load_dataset

(cherry picked from commit 63519eb3)

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