Rename matrix_stack test to avoid name conflicts

  renamed:    test/triqs/arrays/matrix_stack.cpp -> test/triqs/arrays/mat_stack.cpp
  renamed:    test/triqs/arrays/matrix_stack.output -> test/triqs/arrays/mat_stack.output
Fix gf_base_op test

  modified:   test/pytriqs/base/gf_base_op.py
Implement MatrixStack class

This class allows to delegate some operations on the
Green's function data member to the C++ for speed.

new file:   pytriqs/base/gf/local/matrix_stack.pxd
new file:   pytriqs/base/gf/local/matrix_stack.pyx
modified:   pytriqs/base/gf/local/CMakeLists.txt
modified:   triqs/arrays/matrix_stack_view.hpp
Speed up new gf with MatrixStack (issue #28)

This helps mainly the +=, -=, from_L_G_R operations. It addresses
issue #28 and gets back the performances of the old pytacor GF.

  modified:   pytriqs/base/gf/local/gf_generic.py
Improve slightly speed for Omega descriptor

  modified:   pytriqs/base/gf/local/descriptors.py
Improve a bit speed of sumk_lda.py

Some trivial changes have been made. One should spend some
more time profiling this code.

  modified:   pytriqs/dft/sumk_lda.py