kpi: Establish data model for DRB.IPLatDl and DRB.UEActive

3GPP says that this values are averages over collected samples and over observed time period. But if we are to follow 3GPP as-is there is a problem how to aggregate two Measurements corresponding to two different periods into one bigger Mesurement that corresponds to combined time period.

-> Solve that by introducing "statistical profile" types and use them for DRB.IPLatDl and DRB.UEActive;
-> Teach Calc.aggregate to aggregate such statistical profiles via corresponding math.

See individual patches for details.

/cc @paul.graydon