Add support of external function to validate IPv6 checksum

         * @param   p_sourceAddress         Source address, 

         * @param   p_destinationAddress    Destination address

         * @param   p_payloadLength         Upper-Layer Packet Length

         * @param   p_payload               Upper-Layer payload

         * @param   p_nextHdr               Next header value (e.g. 0x3a for ICMPv6)

         */

        function f_computeIPv6CheckSum( 

            in template (value) Ipv6Address p_sourceAddress, 

            in template (value) Ipv6Address p_destinationAddress, 

            in template (value) integer p_payloadLength, 

            in template (value) octetstring p_payload,

            in template (value) UInt8 p_nextHdr 

        ) return Oct2 {

            var Oct2 v_checksum := 'FFFF'O;

            log("*** f_computeIPv6CheckSum: INFO: calling fx_computeIPv6CheckSum() ***");

            v_checksum := fx_computeIPv6CheckSum(p_sourceAddress, p_destinationAddress, p_payloadLength, p_nextHdr);

            v_checksum := fx_computeIPv6CheckSum(p_sourceAddress, p_destinationAddress, p_payloadLength, p_payload, p_nextHdr);

            return v_checksum;

        }