MRG: Forgotten weights arg in barycenter funcs

[hichamjanati]

• Tiny fix: the weights arg was not passed in the general barycenter funcs.
• Loop counter in barycenter_unbalanced was outside the loop -> change while loops to for loops
• Adding 1e-16 to avoid 0 when dual variables are returned was too conservative and causes large differences in the exponentials. changed to 1e-300

[rflamary]

Thank you Hicham !