Add class DataSelection.  IDSClient methods now expect a DataSelection as argument rather then lists of datafileIds, datasetIds, and investigationIds.
Add class DataSelection.  IDSClient methods now expect a DataSelection
as argument rather then lists of datafileIds, datasetIds, and
investigationIds.

git-svn-id: 5b1347ddac5aba1438c637217dfe0bb137609099@319 8efdbd46-c5fb-49ab-9956-99f62928ec21

icatproject/python-icat