Move cppyy/ROOT import and utility functions to a helper module
Additional benefit: the so files and libraries needed for on-the-fly calculations are now only loaded when needed (so I had to add them to the tests).
With these changes, tests pass also with ROOT 6.18/04 (set up with source /cvmfs/sft.cern.ch/lcg/views/LCG_96bpython3/x86_64-centos7-gcc8-opt/setup.sh
)