Fix handling of cell measures during transposes of cubes

[lbdreyer]

Currently cell measures are not handled when you do a transpose of a cube.

>>> c = Cube(np.arange(6).reshape(2,3), 'x_wind')
>>> cm = CellMeasure(np.arange(6).reshape(2,3), measure='area')
>>> c.add_cell_measure(cm, (0,1))
>>> print(c._cell_measures_and_dims)
[[CellMeasure(array([[0, 1, 2],
       [3, 4, 5]]), measure=area, standard_name=None, units=Unit('1')), (0, 1)]]
>>> c.transpose()
>>> print(c._cell_measures_and_dims)
[[CellMeasure(array([[0, 1, 2],
       [3, 4, 5]]), measure=area, standard_name=None, units=Unit('1')), (0, 1)]]

Where I would expect the last line to return:

>>> print(c._cell_measures_and_dims)
[[CellMeasure(array([[0, 1, 2],
       [3, 4, 5]]), measure=area, standard_name=None, units=Unit('1')), (1, 0)]]

i.e. the dims swapped, as it does with auxiliary coords.

This PR includes the handling of the above, plus tests.

[bjlittle]

@lbdreyer Is there an opportunity here to take a step further and combine remap_dim_coord and remap_cube_metadata? e.g., some thing along the lines of:

from collections import Iterable
...
def remap_cube_metadata(coord_and_dims):
    coord, dims = coord_and_dims
    if isinstance(dims, Iterable):
        dims = tuple(dim_mapping[dim] for dim in dims)
    else:
        dims = dim_mapping[dims]
    return coord, dims

Then deriving both self._dim_coords_and_dims, self._aux_coords_and_dims and self._cell_measures_and_dims all call the same helper function?

[bjlittle]

@lbdreyer Minor comment to think about...

Also, travis-ci is failing when it attempts to build the whatsnew. The reason (I think) is that you need to add a .txt to your new whatsnew contribution file.

[bjlittle]

@lbdreyer Awesome, thanks 👍