Add binary_sparsemax_loss and projection_unit_cube

[mblondel]

We should add binary_sparsemax_loss, the binary classification counterpart of multiclass_sparsemax_loss, see section 4.4 of https://arxiv.org/abs/1901.02324. This loss is also known modified Huber loss. The associated mapping is the projection on the unit cube so let's add projection_unit_cube, which is easy to implement using projection_box. Once done, they need to be added to the documentation.

[pharringtonp19]

@mblondel I would be happy to tackle this sometime over the next week or so if no one else has started working on it.

[mblondel]

@pharringtonp19 Sounds good, I don't think any one is working on this right now.