Openfisca-core

* Make sure identic periods are stringified the same way
* _Breaking changes_:
- Change `periods.period` signature.
- It now only accepts strings.
- Restrict the possible inputs for `periods.period`
- The authorized formats are listed in [the doc](https://openfisca.org/doc/key-concepts/periodsinstants.html)
- Deprecate and remove:
- `periods.json_or_python_to_period`
- `periods.make_json_or_python_to_period`

* Move the dummy country to [its own repository](https://github.com/openfisca/openfisca-dummy-country)

* Raise more explicit error when an invalid test case is given
* Raise more friendly error when trying to calculate a variable which doesn't exist
* _Breaking change_: Python exceptions will now be raised when the test case in invalid. Before, only a byriani error was returned. Reusers must therefore adapt their exception handling.

* Add `Entity.to_json` method. Used by OpenFisca-Web-API in `/entity` endpoint in particular.

* Declare `Openfisca-Parsers` as an optional dependency of `Openfisca-Core`.

* Make it mandatory to provide a period when calculating a variable.
- When a computation is requested (with *.calculate_output, entities.__call__, *.calculate[_add|_divide], *.compute[_add|_divide]), the argument `period` is no longer optional.
- Previously, the period of the simulation was used instead of a missing period. It was error-prone, as values would be returned for the wrong period, without any error or warning to alert the formula writer about a likely coding error.