This reverts the part of the commit 1906dc30a5
which introduced modifications in the expr templates of the Green's
functions. The commit had introduced some bugs and this revert
removes them (the expr templates will be taken care of fully in later
commits). This is a temporary fix for issues #18 and #25.
reverted: triqs/gfs/data_proxies.hpp
reverted: triqs/gfs/gf.hpp
reverted: triqs/gfs/gf_expr.hpp