Fix setting up of virtual env and modules for usage of Horovod

Horovod/0.16.2 is needed for parallelized training on Juwels (on Juwels Booster, Horovod is already part of the container!). Unfortunately, the module from the HPC system software stack requires MVAPICH2/2.3.3-GDR which is incompatible with ParastationMPI. The latter is required in conjunction with the netCDF-module for Python. Thus, the module requirements for training and postprocessing now differ and a separation of both has become mandatory. In addition to these adaptions, some other minor corrections related to the virtual environment are included.