[WIP] Sparse output KNN

WIP in early stages

- [x] Implement Cython CSR rowise mode function in sparsefuncs_fast
- [ ] Implement Cython CSR rowise weighted_mode function in sparsefuncs_fast
The [stats.mode is mostly python code](https://github.com/scipy/scipy/blob/v0.14.0/scipy/stats/stats.py#L567).