Finds all common dimension keys in the object including subsets of
    dimensions. If there are is no common subset of dimensions, None
    is returned.

def uniform(obj):
    """
    Finds all common dimension keys in the object including subsets of
    dimensions. If there are is no common subset of dimensions, None
    is returned.
    """
    from .spaces import HoloMap
    dim_groups = obj.traverse(lambda x: tuple(x.kdims),
                              (HoloMap,))
    if dim_groups:
        dgroups = [frozenset(d.name for d in dg) for dg in dim_groups]
        return all(g1 <= g2 or g1 >= g2 for g1 in dgroups for g2 in dgroups)
    return True