value of evaluated cells is not re-used

[cd-mkr]

After evaluating a cell, the variable cell.need_update is set to False (line 98 of evaluator.py) but it is never used. Therefore, when another cell needs the value of the previous cell, it gets re-evaluated again. For simple sheets this is no problem but when the cells depend on other cells which again depend on other cells etc, this quickly slows down the evaluation. To avoid this, line 98 should rather be cell.formula.evaluate = False.