Support zero-based indices about sparsearrays.jl

[rayegun]

So the current design of SparseArrays.jl makes this difficult. The easiest solution is to use CIndices.jl as your index vectors. This means when you unsafe_wrap you should do so with element type CIndex{Int64}. Reinterpreting is expensive so try to avoid that.

The implementation of CIndices is incomplete, it probably needs extensions added for the solvers in SparseArrays.jl, and potentially other libraries. This might be necessary to add to the python interop libraries as well as an optional conversion.

Performance without reinterpreting should be pretty good, with reinterpret it will be pretty bad.

[orenbenkiki]

Thanks for the pointer to CIndices.jl - can you say something in what sense it is incomplete? That is, what operations you expect may fail when using it?

[rayegun]

I don't have any specifics now. I just assume there are some missing operations because it was built piecemeal for this purpose. It is also possible there are functions (like solvers) that convert to Int64 and Int32 that need to be overloaded somehow. KLU.jl should support it now (although I'm not sure if I've tagged the release), I will continue working on the others.