[mpnc] retrieve run-time parameter values instead of compile-time

property values

Several values steering the output as well as the Newton have been
handled inconsistently. While they have been obtained as run-time values
from the parameter tree in other models or parts of the mpnc model, the
compile-time property values have been taken at certain locations. Thus,
despite "advertised" as being run-time, it was not possible to actually
set them run-time. This was also due to the fact that those values have
been set as (private) enums. In order to reach consistency, these values
are member variables now and retrieved run-time.

Reviewed by gruenich.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14228 2fb0f335-1f38-0410-981e-8018bf24f1b0