Add demo program that shows how to build KPI-computing pipeline and to
compute the KPIs. It can be used e.g. as follows:

    $ ./demo/kpidemo.py 60 https://lab.nexedi.com/kirr/misc/raw/162307b9/lte/20221211-overload.xlog

The program comes with extensive comments describing every step.
Please see those comments for details.

The next patch will also add analogous JupyterLab notebook.