Extend sparse linear algebra interface with multiply-add

[l90lpa]

This PR extends Atlas's sparse linear algebra interface with multiply-add functionality.

[l90lpa]

Removed dependency on PR #237

[l90lpa]

Hi @wdeconinck, this PR currently only exposes the simple $y=Ax+y$ interface. I'm wondering if you have an actual need for $y=\alpha Ax+\beta y$ and if not whether we could leave this as follow-on action?

[wdeconinck]

Thanks @l90lpa for this PR. I will review it.

Hi @wdeconinck, this PR currently only exposes the simple y = A x + y interface. I'm wondering if you have an actual need for y = α A x + β y and if not whether we could leave this as follow-on action?

Hi @l90lpa I think it's OK indeed to leave it as a follow-on action if ever needed, but as you say, unlikely.

[wdeconinck]

I think it's really good and cleans up quite a nice chunk in the interpolation adjoint code path, and will improve performance! This could be merged as is a.f.a.i.c

[wdeconinck]

Nice cleanup with new API