ThreadedDiagonalPreconditioner

[lijas]

I took me a while to figure out why more threads did not really speed up my linear sove, since DiagonalPreconditioner is not threaded.

Would it make sense to add something like ThreadedDiagonalPreconditioner? Or is better to make the DiagonalPreconditioner always be threaded?

[mohdibntarek]

I like the always threaded option