Consolidate python package requirements

Currently, python package requirements are spread across multiple locations in the repository (i.e. textual documentation, a Dockerfile and multiple requirements.txt files). Some requirements are not documented at all, or version numbers are missing. Thus, setting up a working build environment from scratch is tedious and even impossible for users not familiar with the python or openPASS ecosystem.

I'd like to have a consolidated version of requirements.txt files in the repository.