stokesncvolumevariables: introduced convenience functions for mass and mole fractions

stokesnclocalresidual: storage term is not set to zero here, this would lead to problems for a derived model model (e.g. nonisothermal stokesnc);
convenience functions are used for mass and mole fractions; small cleanup.

stokesncfluxvariables: use of convenience functions

Reviewed by Markus

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12138 2fb0f335-1f38-0410-981e-8018bf24f1b0