[QUESTIONS/FEATURE] pygmo.decompose_objectives

**Is your feature request related to a problem? Please describe.**

I have three questions about [`pygmo.decompose_objectives`](https://esa.github.io/pygmo2/mo_utils.html#pygmo.decompose_objectives).

1. Does this function assume that the objectives are normalized, i.e., all in the range [0,1]?

2. Should the `ref_point` be the "best" point or the "worst" point? I assume the best point, but in the example it looks like the worst point (assuming minimization).

3. The Tchebycheff decomposition is not "augmented", correct? (See pages 7 and 8 in this [paper](https://link.springer.com/article/10.1007/s11047-018-9685-y).)

**Describe the solution you'd like**

It would be helpful to update the documentation / example accordingly.

It would be helpful to implement the augmented Tchebycheff decomposition.

**Describe alternatives you've considered**

I have implemented the augmented Tchebycheff decomposition myself, but it would still be convenient if it were included in the library.