add COO constructor and update method

[dpo]

The diff makes the change look more complicated than it is. I haven't added SparseMatricesCOO as a dependency; perhaps it's not necessary.

The reason why there are two integer parameters J and I is that size(A) apparently always return Int64s, whereas findnz(A) return integer arrays of type defined by A.

[amontoison]

👍👍

I propose to not parametrize m and n by T and directly set m::Int and n::Int.
In practrice we don't use other integer type.
We can also set m::Integer and n::Integer.

[dpo]

Done.