Improve translation tests

[51-code]

Description

Translation tests currently use protected or public variables in Step classes to check if the translation from DPL was correct. A better way could be to override equals and hashCode methods, then write tests that compare a carefully crafted step to the translation result.

Not as simple of a task as it sounds though, as equality tests will force the objects to be immutable. Right now, AbstractStep uses a mutable field aggregationsUsedBefore, so it has to be refactored. Some regular steps are probably mutable currently as well.

Related to #202 .