Make public API of IcatClient clear

The helper classes are not supposed to be exposed.

So I want to do something like this:

    @property
    def archive_client(self):
        warning.warn("use the direct properties and methods of IcatClient should be used", DeprecationWarning), stacklevel=2)
        return self._archive_client

    @property
    def _archive_client(self):
        if self.__archive_client is None:
            raise RuntimeError("The message queue URL's are missing")
        return self.__archive_client

If they need the object itself for some reason, they can instantiate it separately and they do not need IcatClient.

@bodin Do you think this will cause lots of problems?