UPGRADE recommended!
+ new Schiller Naumann drag law - thx to rq for the contribution!
+ new Mei-lift model- thx to rq for the contribution!
+ new version of big particle voidfraction model applying a Gauss distribution: GaussVoidFraction - thx to rq for the contribution!
+ new functionality to allow adding forces only to DEM integration (not to momentum exchange terms)
+ units of Ksl changed again (hopefully for good) to kg/(m3*s), in the solver Ksl is divided by rho (feedback if rho!=0 bug is fixed now is highly appreciated!)
+ bugfixes