performance test of sparse-matrix vector product added

Tried testing mat-vec product performance with SGSparseMatrix and Eigen::SparseMatrix, used same fixed matrix for both with dim 10^6x10^6, with nnz~3^10^6, nz elements are along the diagonal, the first row and the first col.

On my machine the output is (with -O3)

time	shogun (s)	eigen3 (s)

0	0.940000	0.800000
1	0.940000	0.800000
2	0.940000	0.790000
3	0.940000	0.800000
4	0.940000	0.790000

without -O3, however
time	shogun (s)	eigen3 (s)

0	1.420000	13.450000
1	1.440000	13.460000
2	1.440000	13.480000
3	1.430000	13.470000
4	1.430000	13.470000