Share the quantile function between regression and classification

**Is your feature request related to a problem? Please describe.**
The function that calculates the quantile for regression and classification is not the same.

-> `mapie.utils.compute_quantiles`
-> `mapie.conformity_scores.interface.BaseConformityScore.get_quantile`

**Describe the solution you'd like**
Reuse one of them for the other task, so you only have to manage one function.
