These functions modify the selection or return the currently selected
(highlighted) cases.
iset.selected returns a vector of IDs of all currently
selected cases (in the current iSet)
iset.select changes the selection of the current
iSet. All plots are updated immediately to reflect the new selection.
iset.sel.changed returns TRUE if the selection
has changed since last call to this function.
iset.selectAll selects everything
iset.selectNone clears all selections (yes, not a good name)