Merge pull request #14 from jsantoul/master

utils.py & test_cohort.py Aggregated_present_value tested and OK
Merge pull request #16 from jsantoul/master

New method of Cohorts to generate age classes.
Tried to reproduce the work of C Bonnet.

Fail because of unknown bug with simulation.py file.
More details in Carole_Bonnet.py located in src\scripts.