update abstract method
The observed cell has changed its value.
This method is called after the value of cell
has been
recomputed.
If it is known that the value of cell
has not changed, didChange
is
false otherwise a value of true for didChange
indicates that cell
's
value may have changed (but necessarily has).
cell.value
now holds the cell's new value.
Implementation
void update(ValueCell cell, bool didChange);