Why change the IMTType? Is the a case where the IMT is not in the set described by the IMTType? Currently we have

 `<xs:simpleType name="IMTType">
        <xs:restriction base="xs:string">
            <xs:enumeration value="PGA"/> <!-- peak ground acceleration -->
            <xs:enumeration value="SA"/> <!-- spectral acceleration -->
            <xs:enumeration value="PGV"/> <!-- peak ground velocity -->
            <xs:enumeration value="PGD"/> <!-- peak ground displacement -->
            <xs:enumeration value="IA"/> <!-- arias intensity -->
            <xs:enumeration value="RSD"/> <!-- relative significant duration -->
            <xs:enumeration value="MMI"/> <!-- modified mercalli intensity -->
        </xs:restriction>`