data viewer: display slice as soon as possible
when users ask for several slices the current processing is: waiting until they are all finish to do the synchronization.
But sometimes it make users wait for no much reason.
Maybe we could add a slice
output which is triggered each time a slice is finished. And a viewer could display this slice (dedicated or the data viewer. Need to think about possible 'consequences'). Could add one more combobox or redefine automatically the 'latset slice'