Update installation instruction

Created by: asogaard

Currently we have minimal installation instructions that generally works, but often leads to the odd bug. By suggestion from @MortenHolmRep, we should update our installation process to be simple, robust, and reproducible and document it adequately. This includes considering:

• Should we use conda and pip?
• Can we install pytorch and pytorch-geometric at the same time?
• Should we expand package version requirements to ranges rather than exact ones?
• Should we provide a requirements.txt file? And how would this complement the requirements in setup.py?