Cube builder cannot create custom vertical coordinate

[jrackham-mo]

🐜 Bug Report

When creating a cube with a vertical coordinate, the cube builder cannot handle anything other than 4 levels. This is hard-coded in ants.utils.cube.CubeBuilder.add_model_level_coordinate.

By extension, ants.tests.stock.simple_4d_with_hybrid_height fails when a shape is passed that does not specify 4 vertical levels.

How to reproduce

Step by step guide to reproduce the behaviour:

# use default shape
>>> ants.tests.stock.simple_4d_with_hybrid_height()
<iris 'Cube' of air_temperature / (K) (time: 3; model_level_number: 4; latitude: 5; longitude: 6)>

# try to specify non-default vertical coord, with 5 levels
>>> ants.tests.stock.simple_4d_with_hybrid_height(shape=(3,5,5,6))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../ants/tests/stock.py", line 57, in simple_4d_with_hybrid_height
    builder.add_model_level_coordinate(additional_attributes={"positive": "up"})
  File ".../ants/utils/cube.py", line 238, in add_model_level_coordinate
    self._cube.add_dim_coord(coord, 1)
  File ".../iris/cube.py", line 1409, in add_dim_coord
    self._add_unique_dim_coord(dim_coord, data_dim)
  File ".../iris/cube.py", line 1437, in _add_unique_dim_coord
    raise iris.exceptions.CannotAddError(
iris.exceptions.CannotAddError: Unequal lengths. Cube dimension 1 => 5; coord 'model_level_number' => 4.

Environment

I am using the ANTS 3.1.0 environment.

Version

This bug exists in ANTS 3.1.0 + head of main

Additional Context