Partial collapse of multidimensional auxcoords with bounds

[rcomer]

Since #3028, we can partially collapse a multidimesional auxcoord, which is great. However, it goes wrong if that auxcoord has bounds. Example:

import iris
import numpy as np

print(iris.__version__)

cube = iris.cube.Cube(np.zeros((2, 3)))

coord1 = iris.coords.DimCoord(range(2), long_name='foo')
coord2 = iris.coords.DimCoord(range(3), long_name='bar')

spanning_bounds = np.stack([np.arange(6), np.arange(1, 7)], axis=-1)
spanning_points = np.mean(spanning_bounds, axis=-1)
spanning_coord = iris.coords.AuxCoord(
    spanning_points.reshape(2, 3), bounds=spanning_bounds.reshape(2, 3, 2),
    long_name='spanning')

for i, coord in enumerate([coord1, coord2]):
    cube.add_dim_coord(coord, i)

cube.add_aux_coord(spanning_coord, (0, 1))

print(cube.collapsed('foo', iris.analysis.MEAN))

Output:

2.2.0
[warnings about metadata]
Traceback (most recent call last):
  File "2d_coord_issue.py", line 24, in <module>
    print(cube.collapsed('foo', iris.analysis.MEAN))
  File "[site-packages-path]/iris/cube.py", line 3217, in collapsed
    collapsed_cube.replace_coord(coord.collapsed(local_dims))
  File "[site-packages-path]/iris/cube.py", line 1149, in replace_coord
    self.add_aux_coord(new_coord, dims)
  File "[site-packages-path]/iris/cube.py", line 949, in add_aux_coord
    self._add_unique_aux_coord(coord, data_dims)
  File "[site-packages-path]/iris/cube.py", line 981, in _add_unique_aux_coord
    data_dims = self._check_multi_dim_metadata(coord, data_dims)
  File "[site-packages-path]/iris/cube.py", line 973, in _check_multi_dim_metadata
    metadata.shape[i]))
ValueError: Unequal lengths. Cube dimension 0 => 3; metadata 'spanning' dimension 0 => 2.

[rcomer]

Fixed at #3302.