Converting user libraries of SciPy sparse matrix to sparse array

[dschult]

If folks have libraries who are using SciPy sparse matrices, and you'd like help converting them to run/work with sparse array, this sounds like a nice opportunity to work that out. I wrote a migration guide and have converted a few libraries (working with sklearn now). I suspect an in-person setting could be helpful in this kind of project.

• step 1 is to make the library work with sparse arrays.
• step 2 allow either interface for inputs and choosing from a few options for handling outputs.
• step 3 switch internal creation to sparse array while ensuring "easy" transitions when sparse matrix is removed.

[dschult]

We've been working on the pysal project during the summit.
The PR pysal/libpysal#785 implements "pass 1" of the 2-pass process for migration as described in the Scipy migration guide page.