[test][freeflow][sincos] remove numpy from convergencetest.py
What this MR does / why does DuMux need it:
This removes the numpy include in the convergencetest.py
that caused problems on buildbot and implements its own mean function.
Which issue this MR fixes: fixes #731 (closed)
Special notes for your reviewer: It works on my machine, but please check ;-)
Edited by Simon Emmert