def unravel_group_params(unfolded_parameters_group):
    """Take a dict('group:k'-> p) and return a dict(group -> {k->p})
    """
    parameters_group = {}
    for k, p in unfolded_parameters_group.items():
        group_name, k = k.split(':')
        group_params = parameters_group.get(group_name, {})
        group_params[k] = p
        parameters_group[group_name] = group_params
    return parameters_group