Remove "Incoming" and "Outgoing" concept

[gabesullice]

I think the "incoming"/"outgoing" idea isn't really necessary here. A transformation is a transformation. It shouldn't matter in which context it's being applied. Let something at a higher level decide when to apply a transformation (i.e. on incoming or outgoing data (who cares?))

I think what you want instead, is "reversible" or "undoable". So:

class OneWayTransform extends TransformationBase {
  public function doTransform() {}
}

class TwoWayTransform extends OneWayTransform implements ReversibleTransformationInterface {
  public function undoTransform() {}
}

[e0ipso]

I like this pattern. I'll give it a try. I fear that the documentation of the library will be a bit more complex without the self-explanatory method names.

[e0ipso]

Fixed by #6.