[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
You need to be clear that this is only the mode over nonzeros (or explicit entries). I.e. it is treating the sparse matrix as if it is a masked array.