Add MatSparse::vstack and MatSparse::hstack along with their unit tests.

misc/test/CMakeLists.txt

misc/test/src/C++/unit/test_MatSparse_hstack.cpp.in

misc/test/src/C++/unit/test_MatSparse_vstack.cpp.in

src/faust_linear_operator/CPU/faust_MatSparse.h

src/faust_linear_operator/CPU/faust_MatSparse.hpp