Dummy model sometimes needs altitude coordinate

[lewisblake]

e.g., Obs-only experiments with Earlinet fail because the GriddedData object (dummy_cube) doesn't have altitude as an attribute

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/lustre/storeB/users/lewisb/Python/Evaluations/pyaerocom/pyaerocom/colocation_auto.py", line 830, in run
    coldata = self._run_helper(
              ^^^^^^^^^^^^^^^^^
  File "/lustre/storeB/users/lewisb/Python/Evaluations/pyaerocom/pyaerocom/colocation_auto.py", line 1492, in _run_helper
    coldata = self._colocation_func(**args)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/lustre/storeB/users/lewisb/Python/Evaluations/pyaerocom/pyaerocom/colocation_3d.py", line 376, in colocate_vertical_profile_gridded
    if any(
       ^^^^
  File "/lustre/storeB/users/lewisb/Python/Evaluations/pyaerocom/pyaerocom/colocation_3d.py", line 377, in <genexpr>
    data.altitude.units != Unit(data_ref.metadata[i]["var_info"]["altitude"]["units"])
    ^^^^^^^^^^^^^
  File "/lustre/storeB/users/lewisb/Python/Evaluations/pyaerocom/pyaerocom/griddeddata.py", line 2569, in __getattr__
    return self[attr]
           ~~~~^^^^^^
  File "/lustre/storeB/users/lewisb/Python/Evaluations/pyaerocom/pyaerocom/griddeddata.py", line 2720, in __getitem__
    raise AttributeError(f"GriddedData object has no attribute {indices_or_attr}")
AttributeError: GriddedData object has no attribute altitude