Attempt to make Gauss compatible with python3

Attempt to make a minimum amount of changes to make Gauss compatible with python3. This should not be a major overhaul of the code, which is left to future Gauss built on top of Gaussino. Changes made should keep Gauss fully functional also with python2.

At start, will put it only to lhcb-gaudi-head-py3 slot, if all is fine there, then move to lhcb-gaudi-head slot and only after that to other simulation specific slots.