This patch provides the final building block for E-UTRAN IP Throughput KPI.
It continues

    d102ffaa (drb: Start of the package)
    5bf7dc1c (amari.{drb,xlog}: Provide aggregated DRB statistics in the form of synthetic x.drb_stats message)
    499a7c1b (amari.kpi: Teach LogMeasure to handle x.drb_stats messages)

Quoting those patches

    The scheme to compute E-UTRAN IP Throughput is thus as follows: poll eNB at
    100Hz frequency for `ue_get[stats]` and retrieve information about per-UE/QCI
    streams and the number of transport blocks dl/ul-ed to the UE in question
    during that 10ms frame. Estimate `tx_time` taking into account
    the number of transmitted transport blocks. And estimate whether eNB is congested or
    not based on `dl_use_avg`/`ul_use_avg` taken from `stats`. For the latter we
    also need to poll for `stats` at 100Hz frequency and synchronize
    `ue_get[stats]` and `stats` requests in time so that they both cover the same
    time interval of particular frame.

    Then organize the polling process to provide aggregated statistics in the form of
    new `x.drb_stats` message, and teach `xamari xlog` to save that messages to
    `enb.xlog` together with `stats`.

    Then further adjust `amari.kpi.LogMeasure` and generic `kpi.Measurement`
    and `kpi.Calc` to handle DRB-related data.						<-- NOTE

So here we implement that last noted step:

We add Calc.eutran_ip_throughput() whose implementation is relatively
straightforward as the hard part is done by amari.drb and amari.kpi - in the
Calc we basically need to only divide provided DRB.IPVolDl / DRB.IPTimeDl.