Use sparse CSR matrices to get scikit-learns behaviour. All tests pass

Created by  Gz5AN8tiLGjsQNTG24sqRr5jNodcDoAzyZSgZGoaYTaD  on July 25, 2021
JTA5GVUCWCLJZVNG4FVXFLSV7HSRJQFHQ6CRURD37JGVFPUKMT7QC
Change contents