WINDUP-199: Iteration simplification (pass iteration variable names to c...

...onditions/operations)

## What is the point?

When creating a rule, the user should not care about implementation details in simple cases. The operations and conditions that are nested inside the Iteration are almost always operating on the payloads of the parent Iteration (why would they be nested otherwise?). Therefore, Iteration now sets the variable name to the condition/operation and rule writer does not need to care about the name of the iterated variable anymore.

I also removed the type selection in the AbstractIterationOperation constructor, because the type is already set by the generics (and it should throw exception when the payload type does not match). However this may be for further discussion.

If you like it, I will add tests. 

`mvn clean install` passed.