put a forchCoeff() function into the 1pboxspatialparams. The default value will be set

in the individual boxproperties. This way only the BaseFluxVariables needs to be
set to 
BoxForchheimerFluxVariables<TypeTag>  in order to test the forchheimer
velocity approximation in stead of darcy. 
Reviewed by Bernd


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9377 2fb0f335-1f38-0410-981e-8018bf24f1b0