Enabled outflow conditions (beta version) for the 2p2cni model.

boxspatialparameters: additional empty matrixHeatFlux() method, which uses the face as argument and can handle both, boundary faces and sub-control-volume faces; this is used for the calculation of outflow fluxes only so far and has to be specified in the spatial parameters, if required. 
2p2cnifluxvariables: additional call to matrixHeatFlux(), if we are on a boundary face; changed temperature gradient to a private object with a return function to make it accessible from outside.
boxlocalresidual: removed some superfluous asImp()s.
2p2cnilocalresidual: added computeOutflowValues() method for the evaluation of outflow conditions.
regularizedvangenuchten: corrected typo.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7368 2fb0f335-1f38-0410-981e-8018bf24f1b0