per_capita_present_value and created new function to generate non-constant cohort.
aggregated present value is functionnal and tested but unsure about per_capita
per_capita_generation_present_value test and method are working. Nose reports no problem.
New method for Cohorts : filter to generate a restricted version of some cohort.
Unit test done and working.
the method extract_generation is screwed up and does not work yet, neither does the test.
Debugged the generation_extraction method. It can now handle non-zero ages arguments and border effects.

Now working as intended. The test also running correctly with nose.
Merge pull request #14 from jsantoul/master

utils.py & test_cohort.py Aggregated_present_value tested and OK