Openfisca-core

- Display symbolic values of Enums in /trace and print_computation_log

Breaking changes

- Split the "Entity" class hierarchy (Entity, PersonEntity, GroupEntity) into two parallel hierarchies, representing respectively the abstract, model-level information (classes named Entity etc.) and the population-level information (classes named Population and GroupPopulation)
- As a result, the first parameter passed to a formula is now a Population instance
- Much more detail (and class diagrams) in the PR description
- Remove support from the syntax `some_entity.SOME_ROLE` to access roles (where `some_entity` is the entity passed to a formula).

Migration details

- Use the standard SomeEntity.SOME_ROLE instead. (Where SomeEntity is the capitalized entity or instance, Household.PARENT.)
- Code that relied excessively on internal implementation details of Entity may break, and should be updated to access methods of Entity/Population instead.

- Improve usability of Enum values:
- Details:
- Allow the use of Enum values in comparisons: instead of using `<Enum class>.possible_values` you can simply `import` the Enum class
- Accept Enum values via set_input (same result as the previous point)

Breaking changes

- Require clients to make explicit when input variables cover a range of dates, rather than allowing inputs to be derived from past or future values; also removes the `missing_value` base function

Migration notes

You might need to change your code if any of the following applies:
- you are a model author, and there are variables in your model that use a `base_function` attribute
- your model **or application** uses any non-numeric variables (boolean, date, string or Enum) used in, for which the period on which you define **inputs** does not match the period for which you are requesting **outputs**

Detailed instructions are provided in the [PR description](https://github.com/openfisca/openfisca-core/pull/813).

- Raise an error instead of silently ignoring the input when a user tries to set an input for a variable for several months (or several years), but the variable has no `set_input` declared.

- Apply `flake8-bugbear` recommendations and enforce same in continuous integration.