Smaller modification of Experiment base section

After a discussion with @sanbrock @sbrueck @hnaesstroem @g-sarthakkapoor, we decided about a small change in the Experiment class.

The Experiment has got "steps" property that currently belongs to ExperimentStep(ActivityStep).

• it shouldn't be ActivityStep because this is already a property of the Activity class and it is semantically different as it cannot be a standalone activity, while the ExperimentStep could.
• there is the need of the same structure of the System-SubSystem case.