MAINT prune default value different between celer_path and Lasso

[cruyffturn]

Thank you again for this remarkably fast library.

In the celer_path the default value of prune is 0 while in Lasso, MultiTaskLasso, and GroupLasso it's set to 1 (True).

[mathurinm]

Thanks for pointing this ! It would be better to harmonize indeed. Can you send a PR to harmonize celer_path (set prune default value to True there) and eventually in other functions (Multitask case) ? You can also add an entry in the whatsnew to let users know this has changed