Openfisca-country-template

* Bug fix.
* Impacted periods: all.
* Impacted areas: `openfisca_country_template/variables/benefits.py`.
* Details:
- Fixes `parenting_allowance` formula syntax.
- Uses vectorial computing to return a `parenting_allowance` amount per household.

* Technical improvement.
* Details:
- Forces the installation of the new build each time `make build` is run.
- CircleCI tests against the packaged version of this library.
- When a branch is pushed first time, CircleCI creates a build and caches dependencies.
- Subsequent pushes do not reinstall the build as it is already in cache.
- If the code has been modified in between, changes will be ignored, and tests will fail.

* Minor change.
* Details:
- Fix CHANGELOG versions:
- `3.13.0` was supposed to be `3.12.3`.
- `3.12.0` and `3.12.1` are unpublished.

* Technical improvement.
* Impacted areas: `**/*`.
* Details:
- Make style checks stricter and clearer to help country package developers get started.

* Technical improvement.
* Impacted areas: tests/variables and parameters.
* Details:
- Add a new variable called parenting_allowance to show how group entities and single entities can be used together.
- This variable calls the household_income variable.

> Note: Version `3.12.1` wasn't unpublished by mistake. Please use version `3.12.2` or more recent.