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
• Add typing to data storage #1235
• Add typing to entities #1231
• Add typing to holders #1236
• Add typing to enums #1233
• Add typing to parameters #1237
• Add typing to periods #1229
• Add typing to populations #1238
• Add typing to projectors #1239
• Add typing to reforms #1240
• Add typing to simulations #1241
• Add typing to simulation builder #1242
• Add typing to tax-benefit systems #1243
• Add typing to taxscales #1244
• Add typing to tracers #1245
• Add typing to variables #1246
• Add typing to variables #1246