Skip nullable relations in the natural order.  Warn if the user adds nullable relations in the order.
Skip nullable relations in the natural order.  Warn if the user adds
nullable relations in the order.

git-svn-id: 5b1347ddac5aba1438c637217dfe0bb137609099@604 8efdbd46-c5fb-49ab-9956-99f62928ec21

icatproject/python-icat