ENH: Swap rows in sparsefuncs
Made the following changes
a] Replaced numpy slicing with concatanate
b] Added swap_sparse_column
COSMIT: Replaced ptr1/2 with start/stop
Made the following changes
1. Minor changes to docs
2. Replaced swap with inplace_swap
Merge pull request #3104 from MechCoder/swap_sparse

[MRG+1] ENH: Swap rows in sparsefuncs