Bug fix for the python and PHP XML parsers.

The membersOfInst field in the group DTO class needs a special case
when it is being parsed, since its XML element tag name differs from
the usual name that institution XML elements have.

Added a new unit test to test this, and fixed an existing test to
reflect a change in the Lookup test data, so that all the tests pass
again.

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Imported using git-svn from
https://dev.csi.cam.ac.uk/svn/lookup/trunk@82