Fix infefficient sparsity structure change #3
Description
Inside SysLeft.matrix method setdiag call causes following warning
/home/vhhl/programs/anaconda3/envs/graph_embed/lib/python3.7/site-packages/scipy/sparse/_index.py:127: SparseEfficiencyWarning: Changing the sparsity structure of a csr_matrix is expensive. lil_matrix is more efficient.