form/accumulative: Added "additive" algorithm

The Additive algorithm makes the accumulative strategy actually do what
you THINK the accumulative strategy is doing; namely, add up all the
points, not add up the relative scores of each dimension. In other
words, a grading form that looks like:

Criterion 1, maximum score 100
Criterion 2, maximum score 10

If you gave someone 100 on Criterion 1 and a 0 on Criterion 2, that
person would get a 50% on the old measurement (now called the Mean
algorithm). On the new measurement (the Additive algorithm), they get
100/110, or 91%.

Weights are still respected.