Fix sel for xarray created from single field

[sandorkertesz]

Fixes #554

This PR fixes the issue when sel() did not work on Xarray created from a single GRIB field with the "earthkit" Xarray engine.

E.g. the following code previously failed when calling sel():

import earthkit.data
ds = earthkit.data.from_source("ecmwf-open-data",
source='aws',
model='ifs',
param=["2t"],
step=[24],
)
ds_xr_earthkit = ds.to_xarray()
ds_xr_earthkit['2t'].sel(latitude=35, longitude=-90, method='nearest').values

Now it produces the correct output:

array([281.03421021])

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.13%. Comparing base (ed78a5f) to head (6e0bcdf).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #564   +/-   ##
========================================
  Coverage    90.12%   90.13%           
========================================
  Files          149      149           
  Lines        10950    10960   +10     
  Branches       518      518           
========================================
+ Hits          9869     9879   +10     
  Misses         914      914           
  Partials       167      167           

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