* Refactor formula syntax
- Don't use the object simulation in formula writing. The entity the variable is calculated for is now the first argument of the formula, for instance `person` or `family`.
- The `legislation` is now the (optional) third argument of a formula. It is a function that can be called with a period (or an instant) for argument, and will compute the legislation at `period.start`.
- Don't use `calculate_add` and equivalents. Instead, `add` and `divide` are just given in the `options` argument, for instance `options = [ADD]`.
- Rename `entity_class` to `entity`.
- Don't explicitly use a `calculate` method, but the `person('salary', period)` notation instead.
* Introduce implicit conversions between entities, and improve aggregations formulas.
* Deprecate PersonToEntityColumn and EntityToPersonColumn
* Change the way entities are declared
More information on https://github.com/openfisca/openfisca-core/pull/415