Refactored the function for scaling the size group volume fractions to
better handle situations in which their sum drifts away from unity.
Scaling is now turned on by default, and can be turned off in the
solution dictionary for the population balance.
Additional revision and renaming of *Polydisperse tutorials
Patch contributed by Institute of Fluid Dynamics,
Helmholtz-Zentrum Dresden - Rossendorf (HZDR)
b942ba07e0