fix swaption volatility structures with floating reference date

Hi Luigi, 
it turned out that the previous fix was not enough. The test case is as follows. If you have a swaption cube with floating reference date and shift the evaluation date, both the atm vol structure (which is a member in the cube) and the cube get a notification. However since the cube's termstructure implementation relies on the atm structure (in particular with regards to the reference date) this goes wrong, if the cube is notified before the atm structure. I fixed that and it seems to work correctly, but does not look very nice. Maybe you have a better idea ?
Thanks a lot
Peter