kpi.Calc is calculator to compute KPIs. It can be instantiated on
MeasurementLog and time interval over which to perform computations.
It currently implements calculations for only one "E-RAB Accessibility KPI".

Please see added docstrings and tests for details.

The next patch will also add demo program that uses all kpi.Calc and
other parts of KPI-computation pipeline to build and visualize E-RAB
Accessibility from real data.