New model stokesnc added. Stokesnc will replace the stokes2c model as it is generalised to n components. The employment of stokes2c will produce a deprecated message. The adaption to the stokesnc model is straight forward. Please note that several functions in fluxvariables need an additional input argument compIdx. Associated variables are of size number of components. Changes in configure.ac and Makefile.am for new folder. This commit adds dummy files that copy the history of the stokes2c model. The actual model will be inserted in an extra commit. Reviewed by katherina and klaus.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12076 2fb0f335-1f38-0410-981e-8018bf24f1b0