Openfisca-core

- Limits the range of PyTest versions to < 4.0 to avoid CI crashes caused by 4.0.

- Adds called parameters to Web API `/trace` endpoint
- For a calculated variable, add `parameters` item next to `dependencies` in `/trace` response
- For example:
JSON
{
"income_tax<2017-01>": {
"dependencies": [
"salary<2017-01>"
],
"parameters": {
"taxes.income_tax_rate<2017-01-01>": 0.15
},
"value": [150]
}
}

- Scale parameters are also traced:
json
"parameters": {
"taxes.social_security_contribution<2017-01-01>": {
"0.0": 0.02,
"12400.0": 0.12,
"6000.0": 0.06
}
},

- Exposes `amount` scales through the Web API
- Allows for [AmountTaxScale](https://github.com/openfisca/openfisca-core/blob/2b76b2ae5f684832411694c7c763b2d84c521c3c/openfisca_core/taxscales.py#L119) parameters to be consumed through the Web API
- See for an example this [parameter in openfisca-tunisia](https://github.com/openfisca/openfisca-tunisia/blob/10a15168e0aab5000f6850ad2f7779eba5da0fe0/openfisca_tunisia/parameters/impot_revenu/contribution_budget_etat.yaml)

- Always allow `documentation` property for `ParameterNode`
- Due to a bug, this field was accepted in an `index.yaml` file, but not in a `ParameterNode` located in a single YAML file

- Tests the computation of the average tax rate of a targeted net income, according to the varying gross income.

- Updates the requirements on [numpy](http://www.numpy.org) to permit version 1.16