Since the beginning - since dc1d5481 (kpi: Start of the package)
DRB.IPLatDl.QCI was introduced only in commented form with the following
remark:

        # XXX mean is not good for our model
        # TODO mean -> total + npkt?
        #('DRB.IPLatDl.QCI',                Ttime),     # s         4.4.5.1  32.450:6.3.2   NOTE not ms

The problem here is that if we introduce DRB.IPLatDl.QCI as just Ttime for
average latency, and we have two measurements m1 and m2 with such
DRB.IPLatDl, there is no way to know what DRB.IPLatDl should be
for aggregated measurement - in the aggregated measurement the latency
should be the mean time - averaged for combined periods of m1+m2, over
samples of all transmision bursts. And knowing something already averaged in
period1 and period2 we can compute the average for aggregated measurement
only if we know both initial averages _and_ the number of samples in each
period. That's what the "TODO mean -> total + npkt?" comment was about.

Besides DRB.IPLatDl there are many other values that 3GPP say to be
mean. For example UE.Active and other values. So there is a need to
uniformly represent such averages somehow and that there is a way to
also aggregate the averages for combined measurements.

-> Introduce Stat type, that represents results of statistical profiling
   and use it for DRB.IPLatDl.QCI; Teach Calc.aggregate to handle
   aggregation of such statistical profiles via

            a₁⋅n₁ + a₂·n₂
        A = ─────────────
               n₁ + n₂

   formula.