Merge branch 'issue/grid_function_creator' into 'develop'

src/amdis/GridFunctions.hpp

src/amdis/gridfunctions/AnalyticGridFunction.hpp

src/amdis/gridfunctions/ConstantGridFunction.hpp

src/amdis/gridfunctions/CoordsGridFunction.hpp

src/amdis/gridfunctions/DerivativeGridFunction.hpp

src/amdis/gridfunctions/FunctorGridFunction.hpp

src/amdis/gridfunctions/GridFunctionConcepts.hpp