Compute N x N DFT matrix

Computes DFT matrix for DFT modulation

% dftmat = DFT matrix of order N x N;
% N = Order of the matrix
% computes the DFT matrix of orer N x N;
% DFT matrix is a n x n matrix defined as Wn=[(Wn)^km]
% Wn=exp(-j*2*pi/n)
%Reference: Multirate Systems and Filter banks by P.P Vaidyanathan Pearson Education 1993 appendix A.6