Add test to verify behaviour for scalar height coord for tas in multi…

…-model (#1209)

* Implement a test to establish desired behaviour

* test on 3 cubes

tests/unit/preprocessor/_multimodel/test_multimodel.py

@@ -541,6 +541,26 @@ def test_return_products():
     assert result4 == result2
 
 
+def test_ignore_tas_scalar_height_coord():
+    """Ignore conflicting aux_coords for height in tas."""
+    tas_2m = generate_cube_from_dates("monthly")
+    tas_1p5m = generate_cube_from_dates("monthly")
+
+    for cube, height in zip([tas_2m, tas_1p5m], [2., 1.5]):
+        cube.rename("air_temperature")
+        cube.attributes["short_name"] = "tas"
+        cube.add_aux_coord(
+            iris.coords.AuxCoord([height], var_name="height", units="m"))
+
+    result = mm.multi_model_statistics([tas_2m, tas_2m.copy(), tas_1p5m],
+                                       statistics=['mean'],
+                                       span='full')
+
+    # iris automatically averages the value of the scalar coordinate.
+    assert len(result['mean'].coords("height")) == 1
+    assert result["mean"].coord("height").points == 1.75
+
+
 def test_daily_inconsistent_calendars():
     """Determine behaviour for inconsistent calendars.