Compute tax-scale bracket index and marginal rate

[benjello]

New features

• Introduce AbstractTaxScale.bracket_indices

• Allows for computing the bracket index relevant for any taxable base vector

• Introduce MarginalTaxScale.marginal_rates

• Allows for computing the marginal rate relevant for any taxable base vector

see also openfisca/openfisca-france#1386

[bonjourmauko]

Thanks @benjello !

For this particular pull request, tests aren't enough for me to grasp the intent and the behaviours introduced by it.

Specifically, it'd be great to:

• Add a description with an example to each new method
• Explain the difference between calc and compute

That'll help me a lot to review your contribution! :)

[bonjourmauko]

Rebase + fixup

[bonjourmauko]

Hey @benjello I profited to add some documentation and typing to better understand the behaviours of these classes, before doing a review of the proposed contribution.

[bonjourmauko]

Thanks a lot @benjello, I added some tests with my guesses and there are still two that fail (see in the comments).

Could you take a look?

[bonjourmauko]

I profited of the opportunity to add a user-friendly message:

[bonjourmauko]

LGTM ✨