Validate that None is given

    Parameters
    ----------
    b: {None, 'none'}
        None or string (the case is ignored)

    Returns
    -------
    None

    Raises
    ------
    ValueError

def validate_none(b):
    """Validate that None is given

    Parameters
    ----------
    b: {None, 'none'}
        None or string (the case is ignored)

    Returns
    -------
    None

    Raises
    ------
    ValueError"""
    if isinstance(b, six.string_types):
        b = b.lower()
    if b is None or b == 'none':
        return None
    else:
        raise ValueError('Could not convert "%s" to None' % b)