improve central weighting

	-consistent storage of flux directions in fluxdata -> also applicable for adaptive grids.
	- central in PE for the rare case of potential = 0
	- improve future scalability for 3p3c model
- Introduced flash solver (as had been intended already for dumux 2.1) and removed methods from fluidstates
- properties put in groups
discussed with Philipp

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@8893 2fb0f335-1f38-0410-981e-8018bf24f1b0