NCP model: throw an exception if the minum activity coefficient is <= 0

most likely this will be produces by a negative pressure. If this
happens it is probably better to immediately abort the time
integration anyway...
TPFA: change the way how gradients are calculated

before this, gradients had the direction of the face between two
finite volumes, now they exhibit the direction of the two
FV-centers.

For axis-aligned grids the result is identical for interior faces, but
it is different for boundary faces or if faces are not
axis-aligned. This patch fixes the SPE9 troubles with anisotropic
permeabilities on tilted grids...