- include the property

SpatialParamsForchCoeff 
in all boxmodels. This enables the user to apply a default value for the forchheimer coefficient and merely switch the BaseFluxvariables property. 
- remove include of the model in 2p2c spatialparameters. Otherwise the forchheimer property is multiply defined. headercheck still works. 
- include reference/hint in the forchheiemerspatialparams as how 
to include better-than-default forchheimer coefficients 

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@9384 2fb0f335-1f38-0410-981e-8018bf24f1b0