Openfisca-country-template

* Technical improvement.
* Details:
- Upgrade to Core v20.

* Technical improvement.
* Details:
- Declare package compatible with OpenFisca Core v19.

* Technical improvement.
* Details:
- Declare package compatible with OpenFisca Core v18.

* Tax and benefit system evolution.
* Impacted periods: all.
* Impacted areas: `stats`.
* Details:
- Introduce `total_benefits`.
- Introduce `total_taxes`.

<!-- -->

* Minor change.
* Details:
- Introduce situation examples:
- These examples can be imported with: `from openfisca_country_template.situation_examples import single, couple`.

* Minor change.
* Details:
- Use the technical documentation new address.

* Minor change.
* Details:
- Document entities.