Fix to db8f74e
Add iterator to gf_indices
New version of time_pt

The time points have been changed. Now there are two layers:

- a time segment
- time points that are generated from the time segment

Once a segment is defined it is not necessary to pass beta as an argument. All
points are generated for the same interval. This avoids some confusions.

A new convention is chosen: the point at Nmax == beta == 0. This point can
never be obtained by additions or multiplications. So the largest interval has
Nmax-1. This is good because when a random number is generated with rand(Nmax)
it takes values between 0 and Nmax-1 which is the largest interval.
Fix #165

- The string from h5py needs to be forced to a regular python string,
  since we are not using unicode. Was done for the first case (read_fun)
  but not the other case.
Fix hdf5 formats

- h5 : Python :
- h5 : C++ : add options 'w', 'r', 'a' for C++ file constructor
  to mimic Python
- block gf : clean the hdf5 format and save/load difference between
  Python and C++
- rewrite gf_base_op test to eliminate the h5 file (big).