Support MissingBehaviour argument in CompoundData and CompoundObject

Currently the CompoundData and CompoundObject member() methods take a clunky pair of ( throwExceptions, createIfMissing ) bools, rather than the nice MissingBehaviour enum we use in IndexedIO and SceneCache. Make them use a MissingBehaviour enum instead.

Should we define another enum with the same values, or move the MissingBehaviour enum to the global namespace somewhere? If so, where?
