Memory consumption of latest changes in TQMultiObservable

The changes made to the TQMultiObservable in this MR cause a significant increase in memory usage. This might only be noticeable when running over several channels (systematic variations) but here the introduced inefficiency is significant: when running over 100 variations the memory consumption increased from 1.7Gb to 2.9Gb for me, when the above MR is included.