Added a web service API convenience method to return one or more

(possibly multi-valued) attributes from a person.

This is equivalent to getPerson() with the "fetch" parameter set to
only fetch attributes, except that it returns a list of IbisAttribute
objects, rather than an IbisPerson, and its usage is more obvious.

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Imported using git-svn from
https://dev.csi.cam.ac.uk/svn/lookup/trunk@38