Allow users to specify rules involving the absence of genes now. These
were hard coded in before, instead of being specified in the antecedents
and consequents spreadsheet.
Known bug currently: we are only storing a shell instance when new cells
appear. but the display needs to be updated in some other situations: in
fate color mode, the colors can change at any time. would need to store
a shell for every time step to allow fate colors to remain accurate when
moving backwards in time. not sure if worth it; will consult.