Owned row/column iterators

[mankinskin]

https://github.com/dimforge/nalgebra/issues/834

This is an initial draft for this implementation. I only managed to implement into_row_iter and into_column_iter for DMatrix I think because I could not use Owned<N, R, C> where R: Dim and C: Dim (i.e. R and C were either static or dynamic) because of the implementations of Allocator on DefaultAllocator, which defines Owned:

type Owned<N, R, C> = <DefaultAllocator as Allocator<N, R, C>>::Buffer;

So to use Owned<N, R, C>, R and C have to satisfy the trait bounds for any of the impl Allocator<N, R, C> for DefaultAllocator, but there is no implementation for R: Dim and C: Dim.

There are only

• R = Dynamic, C: Dim (dynamic, either)
• R: DimName, C: DimName (static, static)
• R: Dim, C = Dynamic (either, dynamic)

Maybe there is a way to implement this for R: Dim, C: Dim, but this is the current state.

I would be glad if someone could review this and help me improve this implementation.

[jswrenn]

@mankinskin, this seems like a really interesting design space, so I took a stab at into_row_iter: https://github.com/jswrenn/nalgebra/commit/6e34a2bffe47d4f6724d4364d811808a0737396d (the same approach ought to work for into_col_iter). I don't think I ran into the same issue you did, but it's entirely possible that I did and just haven't noticed.

I wonder if there's some awkwardness here about the meaning of calling into_row_iter on a &Matrix versus on a MatrixSlice.

[sebcrozet]

Thank you @jenanwise and @jswrenn for the suggestions. I believe the suggestion of @jswrenn is a more adequate way of doing this since it will work for all the matrix types and don't require any additional trait bound on the allocator storage output type.