Prepare box models for generalization to cell centered:

- Inherit the model specific LocalResidual and Model from the properties
  BaseLocalResidual and BaseModel instead of BoxLocalResidual and
  BoxModel. 

- Replace fvGeometry.numVertices by fvGeometry.numFAP for the flux
  calculations, or by fvGeometry.numSCV where appropriate.

Reviewed by Christoph.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@8223 2fb0f335-1f38-0410-981e-8018bf24f1b0