New rules to set momentum

The momentum parameter in sinkhorn can be useful to speed up its iterations. This new API allows to define that parameter with a more fine grained tuning.

It is now not only possible to set the momentum (instead of momentum_strategy), it's also possible to set a number of iterations after which that value is refreshed using a simple rule on the decrease of the marginal error of the algorithm. If you are using the momentum_strategy='Lehmann' option then this would be equivalent to momentum=1.0, chg_momentum_from=100