Fixes the issue that elementMapper cannot be updated from the problem file...

Fixes the issue that elementMapper cannot be updated from the problem file needed for adaptive grid methods. The functions returning the elementMapper and vertexMapper are now overloaded with a non constant version of the functions. Reviewed by Bernd.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14040 2fb0f335-1f38-0410-981e-8018bf24f1b0