If t is an atom, return it as a string, otherwise raise InvalidTypeError.

def getAtomChars(t):
    """If t is an atom, return it as a string, otherwise raise InvalidTypeError.
    """
    s = c_char_p()
    if PL_get_atom_chars(t, byref(s)):
        return s.value
    else:
        raise InvalidTypeError("atom")