Add a skeleton for Faust::MatDiag (yet to implement) and a starting test.

misc/test/CMakeLists.txt

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

src/faust_linear_operator/CPU/faust_MatDiag.h

src/faust_linear_operator/CPU/faust_MatDiag.hpp

src/faust_linear_operator/faust_constant.h.in