Add support for the ICAT schema extensions expected in icat.server 5.0

[RKrahl]

There are several schema extensions proposed for icat.server, see for instance icatproject/icat.server#200, icatproject/icat.server#228, icatproject/icat.server#230, and icatproject/icat.server#231. It is expected that there will a release 5.0 of icat.server with a revised schema. Obviously, python-icat must add support for it on the client side.

The entity classes are created dynamically based on the entity information queried from the server and thus, the core of python-icat should support a modified schema right away without modifications. But the icatdump script or more precisely, the icat.dump_queries module will need to take new entity classes and modified relations into account.

[RKrahl]

Obviously this can only be done if icat.server 5.0 has been released, hence the blocked label.