working on data model

Need to figure out how to use sortDescriptor to sort sermons by date
within the sermonsForSelectedSeries getter. Should get rid of "titles
for selected sermons" method perhaps. That method does provide a useful
abstractrion from core data such that it would always return a string
so that if I changed the data model I could still use that method to
return an array of strings and then the
sermonSeriesCollectionViewController wouldn't have to be updated
because all it know is it gets a string! I'll think about this more