Python Virtual Environments
Fixed an issue with the modules, which were shown in the form, but when the job was submitted, it failed to load a module. The reason seems to be the different environment on the Portal, which somehow seems to be inherited by the job.
Also (main thing), added the option to select a Python Virtual Environment, and have it sourced in the job.
OpenComposer only looks in $HOME/venvs for virtual environments, which is also the path described on the wiki. I've also added a link to the wiki article explaining how to setup and use a virtual environment.
