ENH: Add "pure GrB" backend that doesn't rely on SuiteSparse-specific behavior #172

It would be good discipline for python-graphblas to have a backend option that doesn't rely on SuiteSparse-specific functions and behaviors. This will make it easier for python-graphblas to support other GraphBLAS libraries in the future.

I believe we've done a pretty decent job identifying and isolating SuiteSparse-specific functionality. Much is already in .ss attributes. Others are identified by # SS, SuiteSparse-specific comments.

To support such a backend, we can still use SuiteSparse:GraphBLAS, but create a lib object that doesn't have GxB items.