Concatenate with scalar coordinate.

This PR fixes a long standing bug within cube concatenate, that prevents a cube with a single valued dimension coordinate concatenating along that axis to another compatible cube. 

For example, consider the time-series cube (t:10, y:10, x:10). We should be able to concatenate the cube (t:1, y:10, x:10) given compatible metadata and coordinates to the time-series over the first dimension `time`. Previously this was not possible. This PR resolves the issue that prevented such cubes concatenating.
I'm not really convinced of the need to test the individual signature components (dim + aux) here, as the only thing that really matters -- determining the success of a concatenate operation --  is the behaviour of \_CubeSignature.match, which compares everything + returns True/False.
Might it be simpler to just test that ?