renamed DOFVectorView into DiscreteFunction

src/amdis/DOFVector.hpp

src/amdis/ProblemStat.hpp

src/amdis/gridfunctions/CMakeLists.txt

src/amdis/gridfunctions/DOFVectorView.hpp

src/amdis/gridfunctions/DiscreteFunction.hpp

src/amdis/gridfunctions/DiscreteFunction.inc.hpp

src/amdis/gridfunctions/DiscreteLocalFunction.inc.hpp

src/amdis/io/FileWriterCreator.hpp

src/amdis/io/VTKWriter.hpp