Allow formula functions to be methods.
Factorize more Scenario converters.
Allow to create a TaxBenefitSystem with a given legislation_json.
Remove "function_period_unit" from columns and add "period_unit" to formulas.
Add "period" parameter to methods "get_law_period" & "get_variable_period" of formulas.
Add functions and decorators to declare a reference input variable and a reference formula, using a better syntax than build_column... functions.
Add configuration for Nose.
Add a test showing how to use new functions and decorators to declare reference input variables and a reference formulas, using a better syntax than build_column... functions.
Remove debugging code.
Add an example of new syntax for a formula using a function without "self" parameter.
Add helpers to manipulate reforms
Fix flake8 error
Replace calculate methods of holders and formulas to compute methods that return dated holders instead of numpy arrays.
Replace function next_day_instant with next_instant and previous_day_instant with previous_instant.
Replace functions next_instant & previous_instant with (more powerfull) offset_instant. New function "offset" for periods.
Correct tests on dated holders because compute never returns a None dated holder.
Simplify periods.iter_instants and add periods.iter_subperiods.
Improve handling of different periods in formulas and holders.
Add test_holders
Add Makefile for code coverage