Update model_example_blackbox to use python3

[aljavrieling]

There are two tests in ~/OpenDA/tree/master/model_example_blackbox/tests``, 1 for calibration and 1 for kalman filtering. Both tests use ~\stochModel\bin\pollution_model.py`. These tests are run in the test-environment at Deltares and they have been failing for a long time.

From the log-file we can only conclude that the output file is not written. I suspect the problem is that pollution_model.py is still Python2 while the /usr/bin/python on the current agent is Python3. If this is not the (main) problem, it is still a good idea to update the source code to Python3.