* Fix pairwise objective with NDCG metric.
- Allow setting `ndcg_exp_gain` for `rank:pairwise`.
This is useful when using pairwise for objective but ndcg for metric.
- Use the `linalg::Matrix` for storing gradients.
- New API for the custom objective.
- Custom objective for multi-class/multi-target is now required to return the correct shape.
- Custom objective for Python can accept arrays with any strides. (row-major, column-major)
