Since commit r10693, all problems that use a model based on FVPressure had to...

Since commit r10693, all problems that use a model based on FVPressure had to provide an initial(AtPos) function. Default implementations are available in onemodelproblem.hh and impetproblem.hh, but threw an exception. Since many test problems have lived for several years with an initial guess of 0, the default behavior has been changed to not throw an exception and to set the values to 0.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10737 2fb0f335-1f38-0410-981e-8018bf24f1b0