rename files for structured grid creators to make them consistent with the classes which they contain

structuredcubegridcreator.hh -> cubegridcreator.hh
structuredsimplexgridcreator.hh -> simplexgridcreator.hh

also document them properly.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@7765 2fb0f335-1f38-0410-981e-8018bf24f1b0