Continue the impl. of MatDiag.

misc/test/src/C++/test_MatDiag.cpp.in

src/faust_linear_operator/CPU/faust_MatDense.h

src/faust_linear_operator/CPU/faust_MatDense.hpp

src/faust_linear_operator/CPU/faust_MatDiag.h

src/faust_linear_operator/CPU/faust_MatDiag.hpp

src/faust_linear_operator/CPU/faust_MatSparse.h

src/faust_linear_operator/CPU/faust_MatSparse.hpp

src/faust_linear_operator/CPU/faust_Transform.hpp

src/faust_linear_operator/CPU/faust_TransformHelper.hpp