BREAKING CHANGE: cleanup entities

[bonjourmauko]

Depends on #1285

New features

• Add CoreEntity.variables
  • Allows for querying the variables defined for an entity.

Deprecations

• Remove CoreEntity.check_role_validity
  • The method in itself had no relation to entities (it was a static
    method, so no depending neither on an entity class nor an instance). It
    could have been kept as a populations helper, where it was exclusively
    used. Nonetheless, check_role_validity had the peculiar behaviour of
    not failing but when the value of role is not None, that granted it
    in the author's opinion the right to be removed so as to avoid confusion.
• Deprecate class_override arg from entities.build_entity
  • That argument actually did nothing, and linters have been complaining
    about this argument for almost 10 years now. It is only being marked as
    a breaking change for respect to the semantic versioning protocol.
• Deprecate CoreEntity.check_variable_defined_for_entity
  • The feature is still provided by CoreEntity.get_variable when passed
    the optional positional or keyword argument check_existence. In fact,
    the removed method was duplicating and already existing behaviour.
• Deprecate CoreEntity.set_tax_benefit_system
  • The feature is now provided by simple variable assignment:

    entity.tax_benefit_system = tax_benefit_system

  • Likewise, entity.tax_benefit_system becomes part of the public API
    (which implicitly it was as the private marker was being violated).

Technical changes

• Fix failing openfisca-extension-template tests

[benjello]

@bonjourmauko : please take a look at openfisca-survey-manager to see if these deprecations do affect the package.
openfisca-core and openfisca-survey-manager are closely related through usage.
Indeed, a lot of users use country packages with openfisca-survey-manager and may end-up in trouble if their country package works needs the latest version of openfisca-core and while having recent legislation but cannot run with
openfisca-survey-manager.
IMHO, the version bumps should follow openfisca-core -> openfisca-survey-manager -> openfisca-country
And the sooner we check, the best we are prepared.

cc @sandcha @benoit-cty @clallemand @sylvainipp

[bonjourmauko]

We can temporise this one, it is not urgent (it is purely deprecations, so it is expected that it wreaks havoc down the line; yet all deprecations are duplicates with corresponding features).