Openfisca-core

- When there is an empty `index.yaml` in the parameters, ignore it instead of raising an error.

- Fix the internal server error that appeared for the `/trace` and (less frequently) `/calculate` route of the Web API
- This error appeared when a simulation output was a variable of type string


> Note: Versions `24.5.3` and `24.5.4` have been unpublished as they accidentally introduced a breaking change. Please use version `24.5.5` or more recent.

- Ignore W503 to enforce Knuth's style (W504)
- Fix failing entities test
- Household description was outdated

- Further adopt simplified simulation initialisation
- See [729](https://github.com/openfisca/openfisca-core/pull/729)

- In Python, simplify simulation initialisation:

Before:

py
simulation = ...
holder = simulation.persons.get_holder('salary')
holder.set_input('2018-08', [4000])


After:

py
simulation = ...
simulation.set_input('salary', '2018-08', [4000])

- In Python, allow multiline documentation on parameters and variables
- Introduce `documentation` attribute on `ParameterNode`, `Parameter` and `Variable` classes

- In the Web API, expose this documentation as a `documentation` property for parameters, variables and variables' formulas
- on `/parameter` nodes as `/parameter/benefits`
> = python `ParameterNode.documentation`
> = YAML parameter node (`index.yaml`) `documentation` string attribute
- on `/parameter` leafs as `/parameter/benefits/housing_allowance`
> = python `Parameter.documentation`
> = YAML parameter `documentation` string attribute
- on `/variable` as `/variable/housing_allowance`
> = python `Variable.documentation`
- on every `/variable` leaf formula
> = python `Variable` formula **docstring**