Openfisca-core

* Move `base.py` content (file usually located in country packages) to core module `formula_toolbox` so that it can be reused by all countries
* Use `AbstractScenario` if no custom scenario is defined for a tax and benefit sytem

Breaking changes

* Add **mandatory** attribute `definition_period` to variables
* Enforce that inputs provided for a variable match the variable `definition_period`
- If a `set_input` attribute has been defined, automatically cast the input accordingly.
* Change the expected output of a formula
- Only `result` must be returned, instead of `period, result`
* Deprecate and remove:
- `simulation.print_trace`
- `simulation.calculate_add_divide` and `simulation.compute_add_divide`
- `last_duration_last_value` base function
- `variables_name_to_skip` and `use_set_input_hooks` parameters in `scenario.fill_simulation`
- `holders.new_test_case_array`
* Deprecate and forbid `dated_holder.array = ...`
* Raise an error when inconsistent year and month inputs provided for a variable
- Before, the year value would be silently ignored.
* Rename `period.this_month` to `period.first_month`

Technical changes

* Make sure the cache only store values for periods matching the variable `definition_period`
* Use a cache buffer in simulation initialisation when axes are used, to avoid `set_input` conflicts
* Make `DatedHolder` only a wrapper of a variable value for a given period, and no more a dated view of a `Holder`

Documentation

* the attribute `definition_period` is documented here : https://openfisca.org/doc/coding-the-legislation/35_periods.html

* Add `TaxBenefitSystem` doc to the reference doc
- This is transparent for all users

* Improve `openfisca-run-test` script
- Make country package detection more robust (it only worked for packages installed in editable mode)
- Use spaces instead of commas as separator in the script arguments when loading several extensions or reforms (this is more standard)
* Refactor the `scripts` module to seperate the logic specific to yaml test running from the one that can be re-used by any script which needs to build a tax and benefit system.

* Move `json_or_python_to_test_case` from country packages to core
* Breaking change: `scenarios.set_entities_json_id` has been moved, and should not be considered a public function.

* Bug fix : handle the case when CompactNode.name is None.