Make all Python scripts compatible with Python3

Python2 is at the end of its life since quite some time now, it seems a good idea to move all scripts to Python3.

I think the goal is achieved [on this branch](https://github.com/abdomit/DLS-MBF/tree/python3). It was tested and is even used in operation at ESRF, but I cannot guaranty a little script or just part of it has been forgotten.

I took this opportunity to make the tunefit compatible with PythonIoc v 4.5.0 (more convenient to use Python virtual environments)

I created this issue to let other DLS-MBF user aware of this python3 branch, but will not create a pull request on this official DLS-MBF repository.