Modify hubbard_I so that it uses new gf

The test has been adapted as well. Looks OK but some more testing
would be useful.

  modified:   pytriqs/solvers/hubbard_I/solver.py
  modified:   pytriqs/solvers/hubbard_I/solver_base.py
  modified:   pytriqs/solvers/hubbard_I/test/hubbard.output.h5