arrays: fix bug in gemm call

- there was an error when calling gemm for non square matrices
in the computation of the dimension in some cases.
- in the previous version, m was assumed to be the dimension of A,
  while it in fact the first dimension of OP(A) !
- I updated also the matmul test, using non square matrices
(which failed before the patch and is now ok).
- I changed matmul to check result without comparing output,
using a direct computation.

	modified:   matmul.cpp
	deleted:    matmul.output
	modified:   ../../../triqs/arrays/blas_lapack/gemm.hpp