Rename flag to is_permanent.
Convert periods and instants to tuple-based objects.
Reduce over-engineering of average and marginal rates computations.
Simplify use of periods for years.
Simplify columns, formulas, tax-benefit systems & reforms.
Simplify formulas declarations.
Merge remote-tracking branch 'openfisca/master': Simplify columns, formulas, tax-benefit systems & reforms.

Conflicts:
	openfisca_france/model/input_variables/base.py
	openfisca_france/model/input_variables/patrimoine.py
	openfisca_france/model/model.py