For BasicStatisticssNumbers and StatisticsByCategories algorithms.
Adds extra stats (minority, majority, 1st/3rd quartile and IQR) while
avoiding custom statistics calculation code.
Rationale:
- there was a lot of large objects passed by value, so potentially
there's a speed bump from this
- even for implicitly shared classes like QString/QList there's still
a (small) cost for copying the objects when there's no reason to
- it's the right thing to do!
