sincos-test fails because numpy is not available on buildbot

Bug report The test_ff_navierstokes_sincos fails (only!) on buildbot What happened / Problem description: The convergencetest.py includes numpy, but numpy is not available on buildbot. We should try to get rid of numpy in this case. I did not look into the files yet, but the other convergence tests do not need numpy and were desigend to work without numpy.

According to @timok installing numpy on buildbot is not wanted because it is quite large.

Environment:

• Dune version: master
• DuMux version: master
• Others: gcc, Python version: 2.7.9