Remove assignment of interpolator in from_fesom2

We don't want to provide this convenience here, due to the fact that
some variable placements are not supported (e.g. on an edge), but we
don't want to prevent someone who is building a new interpolator from
leveraging this new from_fesom2 method.

Author: fluidnumerics-joe

src/parcels/_core/fieldset.py

@@ -282,6 +282,16 @@ def from_fesom2(ds: ux.UxDataset):
         grid = UxGrid(ds.uxgrid, z=ds.coords["nz"])
         ds = _discover_fesom2_U_and_V(ds)
 
+        for var in ds.data_vars:
+            if "location" not in ds[var].attrs:
+                raise ValueError(
+                    f"Variable {var!r} missing required attribute 'location' to indicate its placement on the mesh."
+                )
+            if ds[var].attrs["location"] not in location_to_interpmethod:
+                raise ValueError(
+                    f"Variable {var!r} has attribute 'location' with unexpected value {ds[var].attrs['location']!r}. Expected one of {list(location_to_interpmethod.keys())}"
+                )
+
         fields = {}
         if "U" in ds.data_vars and "V" in ds.data_vars:
             fields["U"] = Field("U", ds["U"], grid, location_to_interpmethod[ds["U"].attrs.get("location")])