Okay, this is pretty interesting ... I seem to have uncovered a grey area in coordinate signature comparison between cubes for scalar coordinates.

When it comes to comparing the coordinate signature for scalar coordinates, cube merge is really pretty relaxed. It **only** compares scalar coordinate definitions and that's all. It does not consider differences in scalar coordinate meta-data i.e. cell point dtype, bounds dtype, whether both cells are bounded, and the ```circular``` flag for ```DimCoord``` scalar coordinates.

This change here, is a pretty small step towards firming up the scalar coordinate comparison between different cubes in the light of *string* scalar coordinates (which are not bounded).

I don't think this PR is the place to firm up the contract for scalar coordinate comparison. I'm happy to let this sleeping dog lie at the moment, what do you suggest @rhattersley ?