Ginkgo development snapshot for comparison of batched iterative and direct solvers

This is a development snapshot of the batched sparse iterative solver capability in the Ginkgo numerical linear algebra library (https://github.com/ginkgo-project/ginkgo). It can be used to solve a batch of many small sparse linear systems sharing a common sparsity pattern. Benchmarking facilities are included so that comparisons among the different batched solvers can be made, as well as comparison against a vendor-provided batched dense direct solver.