sliced_ell_matrix: Support for Sliced ELL-matrix format (sliced_ell_matrix).

This format is an improved version of the ELL format,
aiming at maximized memory bandwidth for reading matrix values.
Currently only matrix-vector products are supported, no
sparse-matrix-by-dense-matrix products. To be added at some later time.
See paper
"A unified sparse matrix data format for efficient general sparse matrix-vector multiply on modern processors with wide SIMD units"
by Kreutzer et al. for details. We don't use any sorting of rows here.