decoupled 2p: fix initialization in FluxData2P to become independent of

the number of phases and to avoid gcc 4.6's error "assigning to an array
from an initializer list." 

The ordering of the block vectors velocity_ and potential_ is
inconsistent. This should be fixed as soon as possible.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7272 2fb0f335-1f38-0410-981e-8018bf24f1b0