TST Unit test for nb_nll

[ghost]

Reference Issue or PRs

What does your PR implement? Be specific.

1. Improvement of the docstrings of the utils.nb_nll function (maths equations, reference)
2. Adds a unit test to check that the implicit probabilities computed by the function provide the correct moments (which is necessary to achieve a correct implementation, but not sufficient)

[BorisMuzellec]

Hi @mandreux-owkin, thanks for this PR! It's a good idea to clarify what this function does indeed.

There is a little rendering bug with the current docstring (cf below), could you have a look at it when you have time?

[ghost]

Thanks @BorisMuzellec for your review!

There is a little rendering bug with the current docstring (cf below), could you have a look at it when you have time?

Well spotted, thanks, it's fixed. I also changed the values of the tests to make them much faster.

Can you please review again?

[BorisMuzellec]

LGTM, thanks!