Deprecate scale

[andreasnoack]

When 0.4 has been tagged. I think we should deprecate the scale functions (without exclamation marks) in favor of just using multiplication. At least for scaling with a scalar, but I also would prefer A*Diagonal(x) over scale(A,x) for scaling with a vector x.

[tkelman]

Would multiplication by a scalar get dispatched to blas, or have the same level of simd?

[stevengj]

@tkelman, I don't see why it couldn't be implemented that way. I agree that scale! is useful but scale is redundant with *.

[Jutho]

I also think it doesn't matter much whether this is handled by BLAS or by a pure Julia implementation. The dominating factor is probably memory access (and simd), but all the fancy multithreading won't help that much. At least that's the experience I had from some simple comparison a while ago, but I might certainly be misremembering.