Set PStepAheadPredictor as common method in Constraint base class

[EnricoMingo]

I have been working recently on joint limit constraints in acceleration. A common (and actually working) approach to smooth solutions when hitting constraints is to use the PStepAheadPredictor, a constant which scales time.
I now implemented this solution in some constraints, could be the case to add this method as a common method to the constraint class?
If not (so we keep it as optionally implementable by the developer of the constraint) please close this issue.