Created a new class STVariableCollection and moved into it almost everything from STCollection. STCollection is now the ancestor of both STVariableCollection and STPlotCollection (though it doesn't contribute much to either yet). I've beefed up the commenting on both STVariableCollection and STPlotCollection, and removed the selection models from each (to be implemented in views).
I've removed the timeValue and timeUnits information from STPlot as these are features that are currently established in STVariableCollection. Editors that feature a widget for adjusting the plotted time should be transitioned into STCollectionEditors (to be renamed to STVariableCollectionEditors, but it hasn't happened yet).