interpolation of expressions added

dune/amdis/ContextGeometry.hpp

dune/amdis/FileWriter.hpp

dune/amdis/ProblemStat.hpp

dune/amdis/common/IndexSeq.hpp

dune/amdis/common/ScalarTypes.hpp

dune/amdis/common/TypeDefs.hpp

dune/amdis/common/Utility.hpp

dune/amdis/common/ValueCategory.hpp

dune/amdis/linear_algebra/mtl/DOFVector.hpp

dune/amdis/terms/ConstantTerm.hpp

dune/amdis/terms/CoordsTerm.hpp

dune/amdis/terms/DOFVectorTerm.hpp

dune/amdis/terms/FunctorTerm.hpp

dune/amdis/terms/GridViewExpression.hpp

dune/amdis/terms/Interpolation.hpp

dune/amdis/terms/LocalExpression.hpp

dune/amdis/utility/DiscreteFunction.hpp

src/CMakeLists.txt

src/expressions.cc