correctly replay the history in order.


mdg.nodes becomes unordered (dict keys!)
prompt number of the borkenpre node.
hack the stored history.

the detected pre and post conditions are stored in
the history of the session into the history db.
This is done very uglily.

also added an 'echo' isolatemode. if echo is true, the detected pre/post
is printed after cell execution (not very useful)