Starting new method : compute_gen_imbalance.

To be completed.
Merge branch 'master' of https://github.com/openfisca/ga
Started reorganization of the objects and methods.

Moved some irrelavant method in simulation.py
Added new attributes to simulation in order to store the present_value dataframes.
Reorganization of the objects done.

Method compute_gen_imbalance done.
Unit test for the new methods and the moved one to be started.
Changing the class structure of the program.

Creating two new classes that inherit from Cohorts : DataCohorts and AccountingCohorts.
Starting unit test with new classes.

TBC
Unit test done and stable
testing with simulation done and working.
Correcting errors in Carole_Bonnet2.py
Merge pull request #24 from jsantoul/master

Starting new method : compute_gen_imbalance.