Partially implements FS#180 - Inconsistent naming - special status of

saturation. In particular, it deals with the capitalized "S"-indices: 
S0Idx -> s0Idx
SnIdx -> snIdx
SwIdx -> swIdx
SOrX1Idx -> sOrX1Idx
SOrX2Idx -> sOrX2Idx
SnOrXidx -> snOrXIdx
SwOrXIdx -> swOrXIdx 

Reviewed by Christoph. 



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10756 2fb0f335-1f38-0410-981e-8018bf24f1b0