Fix issue of foraging rules being executed for cell types they shouldn't apply to. Ref #4

This should fix issue #4 . For a cell in question, we simply check if the cell type the rule should apply to matches to the cell type this cell had in the previous time step. This should prevent cells being changed several times in one time step by the same disturbance and thus fix problems with changes of one rule being overwritten by another. I think so, at least. Not sure this is the most efficient way to solve this as we still iterate over all disturbances for each rule. But I hope it is effective.