Implemented mean field approximation: it seems to work!
New mean field term
Mean field version of BP_step : BP_step_mf
Some modifications to the mean field computation, and some optimization
of the cython code for solving an Ising chain in the coupled case
(removing tuples speeds up the code).

The mean field case has two implementations doing the same thing, but
with different data structures. Still need to check that they are giving
the same result.
Uncoupled version with interactions only on 4 lines.

The 4 lines may not be very good for real data, until we handle properly
interpolations.
Modified README and setup.py
New version of BP-tomo with uncoupled spins on most lines, runs faster.

The function to build the projection operator now takes a mask as
argument, in order to accept non-circular shapes of the data (as in real
data).
Utiliy routines to transform real data (just starting...).
New demo showing three different versions of BP-tomo