Add binary_sparsemax_loss and projection_unit_cube

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.