amari.kpi: New package with driver for Amarisoft LTE stack to retrieve...

amari.kpi: New package with driver for Amarisoft LTE stack to retrieve KPI-related measurements from logs

amari.kpi provides LogMeasure that takes enb.xlog (TODO and enb.log) as
input, and produces kpi.Measurements on output.

    enb.xlog     ─────────
    ─────────>  │   Log   │
                │         │ ────> []kpi.Measurement
    ─────────>  │ Measure │
    enb.log      ─────────

We read log data organizing periods around stats queries, and for now we
build Measurement from stats' counters. To do so we take δ(stats_prev, stat)
and process it mapping Amarisoft counters to 3GPP ones specified by
kpi.Measurement.

We emit measurement X after reading stats X+2 - i.e. we emit measurement
for a period after reading data covering _next_ period. It is organized
this way to account for init/fini correction:

             fini adjust
            -------------
           '             '
     Sx    v     Sx+1    '   Sx+2
  ────|───────────|───────────|────
       Measurement Measurement
            X          X+1

This approach has following limitations:

- for most of the counters there is no direct mapping in between
  Amarisoft and 3GPP. For example we currently use s1_erab_setup_request for
  ERAB.EstabAddAtt.sum, but this mapping is not strictly correct and will
  break if corresponding S1 E-RAB SETUP REQUEST message contains multiple
  ERABs. The code has corresponding FIXME marks where such approximations
  are used.

- it is not possible to implement init/fini correction precisely. From
  aggregated statistics we only get total amount for a fini value for a
  period - without knowing which part of it corresponds to init events
  from previous period, and which part to init events from current one.
  With that it is only possible to make a reasonable guess and try to
  preserve statistical properties, but not more. See m_initfini in the
  code for details.

- it is possible to handle eNB with single cell only. This limitation
  comes from the fact that in Amarisoft LTE stack S1-related counters
  come as "globals" ones, while e.g. RRC-related counters are "per-cell".
  It is thus not possible to see how much S1 connection establishments
  are associated with one particular cell if there are several of them.

TODO also parse enb.log to fix those issues.

README.rst

@@ -5,5 +5,6 @@
 XLTE repository provides assorted tools and packages with functionality related to LTE:
 
 - `kpi` - process measurements and compute KPIs from them.
+- `amari.kpi` - driver for Amarisoft LTE stack to retrieve KPI-related measurements from logs.
 - `amari.xlog` - extra logging facilities for Amarisoft LTE stack.
 - `xamari` - supplementary tool for managing Amarisoft LTE services.