Bugfix in AMGBackend: take GET_PARAM_FROM_GROUP instead of GET_PROP_VALUE for...

Bugfix in AMGBackend: take GET_PARAM_FROM_GROUP instead of GET_PROP_VALUE for the properties/parameters MaxiTerations, Verbosity and ResidualReduction. Only then, these values can be changed form the input file or command line. Approved by Alex.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11669 2fb0f335-1f38-0410-981e-8018bf24f1b0