Openfisca-extension-template

- Technical improvement: adapt to version `17.0.0` of Openfisca-Core and
version `1.2.4` of Country-Template
- Details:
- Transform XML parameter files to YAML parameter files.
- Gather tests in the directory `tests`. The command `make test` runs only
the tests contained in that directory.
- Add a changelog.

Example Entry

- Tax and benefit system evolution.
- Impacted periods: all.
- Impacted areas:
- `parameters`.
- `variables`.
- Details:
- Import extension from template