Revision of virtual environment set-up on Juwels Booster

While migrating ou source-code to Juwels Booster (e.g. see #26 (closed) and #31 (closed)), we faced a couple of user-defined problems. For instance, the source-code crashed for completely different reasons on Michael's account compared to Bing's and Scralet's account. Closer inspection of the virtual environment set-up revealed that this behaviour can be traced back to the initialization of the virtual environment. In order to access the container's Python packages, the flag --system-site-package has been added with commit a7ba6c1c. However, this also has the drawback that the user-specific Python site-packages (located under $HOME/.local/lib/python3.6/site-packages/) become accessable from the virtual environment, thus integrating the undesired user-specific dependancy.

An alternative and bette approach is to abandon the above mentioned flag and to extend PYTHONPATH by the path to the container's site packages (i.e. export PYTHONPATH=/usr/local/lib/python3.6/dist-packages:PYTHONPATH) while ensuring installation of all missing Python packages such as netCDF4-1.5.3 with requirements_booster.txt. In addition to include those adaptions, we agreed on fixing all package-versions to prevent from potential incompatibilites which may arise with newer Python package versions.