Add sparse client using scipy.sparse

[danielballan]

At the moment scipy.sparse.coo_array is limited to 1- and 2-dimensional arrays. Work is in progress to expand COO to N-dimensional. When that lands, we should an experimental Python client object for sparse arrays using scipy.sparse.

The only we currently have uses the PyData sparse library, which has had N-dimensional COO arrays for awhile.

Let's keep eye on the progress on these issues:

• ENH: sparse: Add nD COO support for matmul, dot and tensordot scipy/scipy#21435
• ENH: sparse: add nD support for element-wise and reduction operations scipy/scipy#21613