Returns a matrix that is the column-wise convolution of
g with h,
where g and h are integer, real, double-precision,
or complex matrices with the same column dimension.
If ng and nh are the number of rows in the matrices
g and h, respectively,
the (k,j)-th element of the return value is
----- / 0 if k-i+1 < 1
> gH where H = < 0 if k-i+1 > nh
----- i,jk-i+1,jk-i+1,j \ h otherwise i = 1 k-i+1,j The return value is a matrix with row dimension equal to
ng + nh - 1 with the same type and column dimension as g.
An FFT is used to compute the convolution.
function computes a similar value directly.