Added the possibility to replace one of the two component mass balances

by a total mass balance. This was done by adding a property
ReplaceCompIdx, which defines which equation is replaced. 
NOTE: adjust the boundary conditions and source/sink values if one
component balance is replaced.
If ReplaceCompIdx is >= numComponents (set as default), the model does the same as
before. This change was neccessary for the model coupling of 2cnistokes
(uses a total mass balance) and 2p2cni.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6195 2fb0f335-1f38-0410-981e-8018bf24f1b0