[implicit 2p2c] add possibility to choose between using a

ConstraintSolver or explicit calculation of secondary variables

For some problem settings, an explicit calculation of the secondary
variables proves to be more stable than using a ConstraintSolver. This
introduces the possiblity to choose between the two variants analogous
to 3p3c.

Reviewed by Bernd.


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14119 2fb0f335-1f38-0410-981e-8018bf24f1b0