FieldCalculations: Calculate % A of B

Usecase: Calculate the % the advertising costs are of the revenues.

I've also build a simple addition and subtraction.

The FieldCalculations live aside of the Aggregator. I'm not really happy with this, but I don`t have the time at the moment to refactor the whole calculation engine.

The problem is, that the FieldCalculation need the results of other Aggregator fields. So it needs a way to query those data on demand. The Aggregator itself get its data dependencies pushed into it. So its not easy to come up with a common interface...