GEMM / GEMV et al calling scal

[wolfv]

Currently, GEMM and GEMV (and related functions) call scal which is not totally correct. In the original BLAS implementation, first it is checked wether the scaling parameter is 0 or not.
If the scaling parameter is 0, the array is 0 initialized.

This leads to such problems as spurious nan's when computing the result into an uninitialized block of memory for xtensor-blas users.

Ref: xtensor-stack/xtensor-blas#75