Changed coord_dims to accept a coord name as well as a coord

Minor change so that iris.cube.Cube.coord_dims() can accept a coordinate object, or a coord name string, like most of (all?) the other coordinate accepting functions.

UPDATE: This is being achieved by changing the behaviour of ```Cube.coord``` and ```Cube.coords``` to accept a ```name_or_coord``` which is then type checked, rather than the separate ```name``` and ```coord``` kwargs