Improve package's typing

[bonjourmauko]

Hi there!

Here is what I did:

I used a function expecting an int as argument, with a str argument.

Here is what I expected to happen:

To get an error from make check-types.

Here is what actually happened:

Nothing.

Context:

Type-hinting is an inexpensive way of :

• Documenting the code
• Building the documentation
• Finding an correcting bugs —I've spotted quite some already
• Helping contributors/maintainers enforce contracts, with zero runtime impact to reusers
• Finally, they're a just part of the standard core of Python, like using a list or concatenating a string

TODO

• Add typing to commons #1230
• [2/17] Document the data_storage module #1167
• Add typing to entities #1231
• 4. Improve openfisca_core.holders typing
• docs: fix doc & types in enums #1224
• 6. Improve openfisca_core.parameters typing
• Add typing to enums #1233
• 8. Improve openfisca_core.populations typing
• [9/17] Fix doc & type definitions in the projectors module #1221
• 10. Improve openfisca_core.reforms typing
• 11. Improve openfisca_core.simulation_builder typing
• 12. Improve openfisca_core.simulations typing
• 13. Improve openfisca_core.taxbenefitsystems typing
• 14. Improve openfisca_core.taxscales typing
• 15. Improve openfisca_core.tracers typing
• 16. Improve openfisca_core.variables typing
• 17. Improve openfisca_web_api.loader typing