Categorisation of 2D coords

This allows categorisation of coordinates of more than 1 dimension. This is achieved by calling `np.vectorize` on the input `category_function` to the `iris.coord_categorisation` function `add_categorised_coord`, which allows for _n_-dimensional coordinates to be inputted as an argument to `category_function`. Note that due to numpy/numpy#3270 a temporary special-case workaround for long strings has been added.

Unit test for `iris.coord_categorisation.add_categorised_coord` added.