Openfisca-core

Technical changes

- In tests, extract `CountryTaxBenefitSystem` to a fixture reusable by all the tests in the test suite.
- It allows for a better reusability of test scenarios available to new contributors.
- To mitigate possible performance issues, by default the fixture is initialised once per test module.

Technical improvements

- Change logging.warning to warnings.warn to allow users of the package to hide them.

Bug fix

- Repair expansion of axes on a variable given as input
- When expanding axes, the expected behaviour is to override any input value for the requested variable and period
- As longs as we passed some input for a variable on a period, it was not being overrode, creating a NumPy's error (boradcasting)
- By additionally checking that an input was given, now we make that the array has the correct shape by constructing it with NumPy's tile with a shape equal to the number of the axis expansion count requested.

Technical improvements

- Render all OpenFisca Core components modular and sandboxed
- Allows for simpler contribution
- Allows for better unit testing
- Allows for dealing with circular dependencies
- Allows for more explicit contracts between components

Future deprecations

- Reorganise `formula_helpers`, `memory_config`, `rates`, `simulation_builder`.
- Their functionalities are still there, just moved around
- Transitional `__init__.py` files added to make transition smooth

- Rename of some errors for consistency
- Added the suffix `Error` to all errors
- They're still exposed publicly as they were before

Bug fix

- [Web API] Gracefully handle unexpected errors
- The exception signature expected by the internal server error handler was not the good one
- Henceforth no response was being given to the user, when a 500 with an explanation was expected

New features

- Introduce max_spiral_loops option in YAML test files
- Allows for control of spiral depth for every test