Reproducing C Bonnet's work with truncated data.

Results are plausible. Two downsides for now :
unborns in 2007 have a lower net_transfer present value than the 2007 newborns. Need to identify where this discrepancy comes from.
New method of Cohorts() : compute_ipl.

The method has to be reviewed to check if nothing has been forgotten for the calculation of the IPL.
Merge pull request #21 from jsantoul/master

Attempt to recreate C Bonnet's work with truncated data