[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
This function as it is right now works as a mode function, it is slow however because of what you pointed out. I will focus on getting a functional PR  first before coming back to optimize this using Joels comments.