Remove scaler mulitply

[hiccup7]

OpenBLAS v0.2.15 will improve the performance of many functions, and I am waiting for this release before comparing some functions with Yeppp. Today, I found that the Yeppp scaler multiply is slower than the base that uses OpenBLAS v0.2.14:

blas_set_num_threads(CPU_CORES)
using Yeppp

const n = 100000
const reps = 200000
const a = 2.0
const x = ones(Float64, n)
const res = similar(x)
@time for k=1:reps; Yeppp.multiply!(res, x, a); end
@time for k=1:reps; Yeppp.multiply!(res, x, a); end
#11.0 seconds, single-threaded

@time for k=1:reps; scale!(x, a); end
#6.9 seconds, single-threaded

To avoid confusion from a Yeppp function that is slower than the base, I propose to remove the Yeppp.multiply!() method using a scaler argument. Good to do this before Yeppp gets widely used in Julia.

[Maratyszcza]

Right, multiplication by scalar doesn't have optimized kernels in Yeppp 1.0.0

BTW, optimized kernels are documented in Yeppp! C API reference

[ViralBShah]

Good point.

[ViralBShah]

@Maratyszcza Which are the Yeppp functions that we really should be providing here, in addition to what we have, where Yeppp will give serious gains? What else should we remove, except for scalar operations?

[panlanfeng]

The scalar operations and negate! function are not optimized. I agree with removing them until a new version of Yeppp! is released.

[ViralBShah]

Please go for it if you can.