Implement softDTW divergences

[rtavenar]

As discussed in #307, several similarity measures for time series (based on softDTW, namely "sharp softDTW" and "softDTW divergence") are introduced in [1] and it would make sense to make them available in tslearn.

As written in #307, "the heaviest piece of work there would be to have careful docs that would prevent end-users from getting confused about all these variants".

[1] Mathieu Blondel, Arthur Mensch, Jean-Philippe Vert. Differentiable Divergences Between Time Series