LinkRefByValueStructure: validate that fromPointRef and toPointRef is uniform with what we have in other structures, in which they are different elements

[skinkie]

        <xsd:complexType name="LinkRefByValueStructure">
                <xsd:annotation>
                        <xsd:documentation>Type for a reference to a LINK.</xsd:documentation> 
                </xsd:annotation>
                <xsd:attribute name="nameOfClass" type="NameOfClass">
                        <xsd:annotation>
                                <xsd:documentation>Name of class of LINK.</xsd:documentation>
                        </xsd:annotation>
                </xsd:attribute>
                <xsd:attributeGroup ref="ReferenceModificationDetailsGroup"/>
                <xsd:attribute name="fromPointRef" type="PointIdType" use="required">
                        <xsd:annotation>
                                <xsd:documentation>Identifier of a POINT. at which LINK starts.</xsd:documentation>
                        </xsd:annotation>
                </xsd:attribute>
                <xsd:attribute name="toPointRef" type="PointIdType" use="required">
                        <xsd:annotation>
                                <xsd:documentation>Identifier of a POINT. at which LINK ends.</xsd:documentation>
                        </xsd:annotation>
                </xsd:attribute>
                <xsd:attribute name="nameOfPointRefClass" type="NameOfClass">
                        <xsd:annotation>
                                <xsd:documentation>Class of POINT referenced by LINK.</xsd:documentation>
                        </xsd:annotation>
                </xsd:attribute>
                <xsd:attribute name="typeOfLinkRef" type="TypeOfLinkIdType">
                        <xsd:annotation>
                                <xsd:documentation>Reference to a TYPE OF LINK.</xsd:documentation>
                        </xsd:annotation>
                </xsd:attribute>
        </xsd:complexType>