Warn coord present

Currently iris will raise an error if you try and add a new coord categorisation that has the same name as a coordinate already present on the cube. I find myself littering my analysis code with the following type of thing:

    try:
        iris.coord_categorisation.add_month(cube, 'time')
    except ValueError as e:
        cube.remove_coord('month')
        iris.coord_categorisation.add_month(cube, 'time')
        warnings.warn("Overwrote coord: %s" % str(e))

I've changed the code so that, instead of raising an error, it just overwrites the coordinate and raises a warning.