[DSML Transformator] Clear the Expression holder for repetetive transform() calls
Current behavior
The Expression holder is currently reused if multiple transform() calls are issued for the same DSMLtoSMTTransformator.
Fix
The holder has to be cleared when the transform() method is called to
avoid reusing constraints / objectives that are no longer in the passed
ExplorationSpecification.
Clearing the holder cannot be done on method exit since the transformed
expressions must still be present for the model conversion that is
called separately.
(from redmine: issue id 3756, created on 2019-07-01, closed on 2019-07-02)