[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
I believe this function has support for the zeros in a sparse matrix, we first do the one dimensional mode on the data in the row, then we count the number of zeros with ```nnz``` to see if the number of zeros is larger then the most frequent element from the mode calculation and return zero if so.