remove unnecessary trait bounds in matrix conversions.

[taketo1024]

Removed some of the unnecessary trait bounds in matrix conversions.

[Andlon]

I was surprised to see that we can remove ClosedAdd from COO->CSR conversion, since if there are duplicates in the COO format, we add them together. Digging a little into it, I see that convert_coo_cs, which convert_coo_csr calls, somehow calls combine_duplicates with std::ops::Add::add, yet it has no bounds for Add. I'm perplexed - what am I missing here?

[Andlon]

Ah, num_traits::Zero requires Add. That is uh, not so obvious IMO. Well, in any case, we can remove these bounds. But since we try to provide the same API for CsrMatrix and CscMatrix, can you please add the same changes to CscMatrix @taketo1024? That would be great!