RETM: Allow subclasses to recompute rows (NYI: event firing)

I'm hoping for the sweepstakes editor to pull the sweepstakes that would
be assigned based solely on the logic for the events and allow the user
to enter an "offset" from this value. The sum would then be dynamically
computed. This is an example of a use case for this method.

I should probably make some of these methods final.
RETM: Fire events when computing rows

CAVEAT: This could lead to infinite recursion. To solve this, I'll add a
check in the tableChanged method that we haven't already computed this
field. I might save all of the editable fields whenever we compute a row
and, before computing the row, check that we haven't already computed it
for the given values. (Of course, if subclassers mess with editable
fields when recomputing, they should take care not to recurse
infinitely.)

computeRows(1, 1)
-> computeRow(1)
-> fireTableRowsUpdated(1, 1)
(I'll add an event listener later)
-> tableChanged(relevant TableModelEvent)
(presumably)
-> computeRows(relevant TableModelEvent)
-> computeRows(1, 1)
RETM.comRow(Obj[]): Add note about infinite recursion