Check that qualifiers are expected values.

def _chk_qualifier(self, qualifiers):
        """Check that qualifiers are expected values."""
        # http://geneontology.org/page/go-annotation-conventions#qual
        for qual in qualifiers:
            assert qual in self.exp_qualifiers, "UNEXPECTED QUALIFIER({Q}): {F}".format(
                Q=qual, F=self.filename)