Improve dot performance (#389)

- improves dot for two COO arrays, fixes #331 
    - I used the same algorithm that scipy uses.
    - It's a bit slower than scipy because to ensure sorted coordinates, we have to do some sorting.

- add dot for GCXS arrays
    -  Uses the same algorithm.
    - Works for `csr @ csr` and  `csc @ csc`
    - I found that the `csr @ csc` dot product-based algorithm was just to slow, so we convert one of the arrays. 

- add io for GCXS arrays

Author: daletovar

sparse/__init__.py

@@ -1,4 +1,4 @@
-from ._coo import COO
+from ._coo import COO, as_coo
 from ._dok import DOK
 from ._sparse_array import SparseArray
 from ._utils import random