Conversion of lecture notes, practice and homework problems to Python 3

Converted most of the code to be compatible with Python 3.

Done as separate branch to keep the Python-2-branch unaffected for now. Unfortunately github attempts to merge into current master, can this be circumvented when merged?

Things still left out:
* lectures/running_Python_code
* everything dealing with remote resources:
  - lectures/accessing_remote_ressources
  - practice problem dealing with remote ressources