Develop a better class hierarch separating madevent and standalone in output.py

[valassi]

Develop a better class hierarchy separating madeven and standalone in output.py

See Olivier's comment in #813 (comment) and the discussion there.

Assigned mainly on Olivier to define what would be the correct approach here

[oliviermattelaer]

So this is now implemented in #813 and waiting for your review.
So I'm closing this.