Array conventions: support CSR

it is quite necessary to provide conversion from our BaseMatrix format and scipy array/matrix formats. More details to come.

See [`petgraph`'s CSR](https://github.com/petgraph/petgraph/blob/ffb085f865b390d12eef7d6ab72a5a7d84a644c5/src/csr.rs#L47) as example.