[implicitadapt] Make initializationindicator work for all types of

boundary/source function available for implicit problems.

The init indicator used to assume to have an *AtPos method available.
However, these methods are not always available. The new inidicator uses
the respective highest method in hierarchy.

Reviewed by martins 


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@15375 2fb0f335-1f38-0410-981e-8018bf24f1b0