Openfisca-core

Minor Change

* Enhance logs
- Replace prints by proper logs
- Give clear and concise feedback to users when a formula cannot compute.

* Enable API monitoring with Piwik
- See the [documentation](https://github.com/openfisca/openfisca-core/#tracker)

Breaking changes - [545](https://github.com/openfisca/openfisca-core/pull/545)

* Deprecate `parsers`
- They can no longer be installed through `pip install openfisca_core[parsers]`
- They can still install them directly with `pip install openfisca_parsers`, but they won't be maintained.

Bug fix

- Make `missing_value` base function compatible with v4 syntax and extra params

Breaking changes - [525](https://github.com/openfisca/openfisca-core/pull/525)

* Rename `Variable` attribute `url` to `reference`
- This attribute is the legislative reference of a variable.
- As previously, this attribute can be a string, or a list of strings.
* Rename `Variable` attribute `reference` to `baseline_variable`
- This attribute is, for a variable defined in a reform, the baseline variable the reform variable is replacing.
* Remove variable attribute `law_reference`
* Rename `TaxBenefitSystem.reference` to `TaxBenefitSystem.baseline`
* Rename `TaxBenefitSystem.get_reference_compact_legislation` to `TaxBenefitSystem.get_baseline_compact_legislation`
* Rename `Simulation.reference_compact_legislation_by_instant_cache` to `Simulation.baseline_compact_legislation_by_instant_cache`
* Rename `Simulation.get_reference_compact_legislation` to `Simulation.get_baseline_compact_legislation`
* Rename parameter `reference` of `AbstractScenario.new_simulation()` to `use_baseline`
* Rename parameter `reference` of `Simulation.legislation_at()` to `use_baseline`

New features

- In the preview API `/calculate` route
- Handle roles with no plural
- Force all persons to be allocated to all entities
- Improve error messages
- Detect unexpected entity errors first
- Document new simulation and entities constructors.