Fix issues in gf and add enforce_discontinuity

  modified:   pytriqs/base/gf/local/functions.pxd
  modified:   pytriqs/base/gf/local/gf.pyx
  modified:   pytriqs/base/gf/local/lazy_expressions.py
  modified:   pytriqs/base/gf/local/legendre.pxd
  modified:   pytriqs/base/gf/local/legendre.pyx
  modified:   triqs/gf/local/functions.cpp
  modified:   triqs/gf/local/functions.hpp
Turn to new gf in python part of ctqmc_hyb

For now the bethe test works. For cdmft one needs to go
correct some other python parts (sumk).

  modified:   pytriqs/solvers/ctqmc_hyb/ctqmc_solver.pyx
  modified:   pytriqs/solvers/ctqmc_hyb/models.py
  modified:   pytriqs/solvers/ctqmc_hyb/test/cdmft_4_sites.py
  modified:   pytriqs/solvers/ctqmc_hyb/test/single_site_bethe.py