Similarly to DRB.IPLatDl 3GPP says to use average number of active UEs
during a period for DRB.UEActive. Which means that we'll also need to
use statistical profile for this value to be able to aggregate
measurements. But contrary to DRB.IPLatDl, DRB.UEActive should be
statistical profile with time-based sampling because the averaging
happened over time, not over arbitrary general samples.

-> Introduce StatT type, that is similar to Stat, to represent such
   time-based profiling and use it to define DRB.UEActive; Teach
   Calc.aggregate to handle aggregation of such time-based statistical
   profiles via

	    a₁⋅δt₁ + a₂·δt₂
	A = ───────────────
	       δt₁ + δt₂

   formula.

This patch is partly based on the following patch by Paul Graydon:
eb9f1fa9