arrays : add matrix_stack_view

- add matrix_stack_view (a stack of matrix in a 3d array)
to accelerate cython code on these operations.
- added a C++ test for it

- clef : added == operation which was missing
- matrix_view_proxy : added the << operator on ostream to print the
  view..

	new file:   ../test/triqs/arrays/matrix_stack.cpp
	new file:   ../test/triqs/arrays/matrix_stack.output
	new file:   ../triqs/arrays/matrix_stack_view.hpp
	modified:   ../triqs/arrays/matrix_view_proxy.hpp
	modified:   ../triqs/clef/clef.hpp