line_search_armijo returns alpha > 1

[octavian-ganea]

line_search_armijo can return alpha > 1 because that's what the scipy function was doing. However, this affects the cg() function used in both GW and FGW functions and returns transport plans with negative entries when armijo=True.

[rflamary]

This is indeed a problem.

If you want to do a PR to handle this case (basically return min(1,alpha)) you are welcome. Else we will get around to it in the following days.

[rflamary]

Closed in #189