Combine ERA5 and ERA5T data

Requesting cutout data spanning recent (ERA5T)
and data older than ~3 months (ERA5) results in an
additional dimension in `cutout.data`, called `expver`,
which `atlite` currently cannot handle gracefully.

This change collapses the two dimensions into a single dimension.

See discussion in PyPSA#190

RELEASE_NOTES.rst

@@ -19,6 +19,9 @@ Upcoming Release
   See `#256 <https://github.com/PyPSA/atlite/issues/256#issuecomment-1271446531>`_ for details.
 * Bugfix: The hydro inflow calculation was relying on a wrong distance calculation in `atlite.hydro.shift_and_aggregate_runoff_for_plants`. This is now fixed.
 * Add a reference to the PyPSA ecosystem community server hosted on `Discord <https://discord.gg/AnuJBk23FU>`_
+* Bugfix: building cutouts spanning the most recent few months resulted in errors due to the
+  mixing of `ERA5` and `ERA5T` data returned from the CDSAPI.
+  See `#190 <https://github.com/PyPSA/atlite/issues/190>`_ for details.
 
 Version 0.2.9
 =============

atlite/datasets/era5.py

@@ -91,6 +91,14 @@ def _rename_and_clean_coords(ds, add_lon_lat=True):
     ds = maybe_swap_spatial_dims(ds)
     if add_lon_lat:
         ds = ds.assign_coords(lon=ds.coords["x"], lat=ds.coords["y"])
+
+    # Combine ERA5 and ERA5T data into a single dimension.
+    # See https://github.com/PyPSA/atlite/issues/190
+    if "expver" in ds.dims.keys():
+        # expver=1 is ERA5 data, expver=5 is ERA5T data
+        # This combines both by filling in NaNs from ERA5 data with values from ERA5T.
+        ds = ds.sel(expver=1).combine_first(ds.sel(expver=5))
+
     return ds