Resolve "TQMultiObservable should lazily evaluate subObservables"

Release notes

Lazy evaluation of components in TQMultiObservable: if using Root >6.06 (i.e. Rel21+) TQMultiObservable now only evaluates subobservables when really needed. This allows, e.g., for easier handling of quantities that are only defined under certain conditions like DYjj requiring at least two jets in the event. For example the expression '[nJets]>1 ? [DYjj] : -999.' will only evaluate the DYjj observable if the value of the nJets observable is >1 for the respective event.

Details

see issue closed by this MR. Also note again that this only affects things when compiling against ROOT 6.06+ as the TFormula constructor needed to make this work was only introduced in 6.06!

Closes #162 (closed)