ENH: Add "Celsius" alias (#336)

On some instance of E-OBS dataset, the "units" is Celsius instead of the standard "celsius".

.gitignore

@@ -130,3 +130,6 @@ dmypy.json
 
 # Pyre type checker
 .pyre/
+
+# Pycharm config
+.idea

cf_xarray/tests/test_units.py

@@ -73,3 +73,11 @@ def test_udunits_format(units, expected):
     u = ureg.parse_units(units)
 
     assert f"{u:cf}" == expected
+
+
+@pytest.mark.parametrize(
+    "alias",
+    [ureg("Celsius"), ureg("degC"), ureg("C"), ureg("deg_C"), ureg("degrees_Celsius")],
+)
+def test_temperature_aliases(alias):
+    assert alias == ureg("celsius")

cf_xarray/units.py

@@ -91,7 +91,7 @@ def repl(m):
 units.define("year = 365.25 * day = yr")
 
 # Define commonly encountered units not defined by pint
-units.define("@alias degC = C = deg_C = degrees_Celsius")
+units.define("@alias degC = C = deg_C = Celsius = degrees_Celsius")
 units.define("@alias degK = deg_K")
 units.define("@alias day = d")
 units.define("@alias hour = h")  # Not the Planck constant...