Discrete frequencies:
  Call: f=disc_freq(N,dt);
  N is the number of elements contained in the signal
  dt is the temporal or spatial sampling interval
 
  Knud S. Cordua, 2010
 
  Verification by symetri:
  
  dt=0.01;
  N=2^nextpow2(1000)+1;
  Tp=1;
  w=gausswavelet(Tp,N,dt);
  wf=fft(w);
  wf=fftshift(wf);
  pwr=real(wf).^2+imag(wf).^2;
  f=disc_freq(N,dt);
  figure, plot(f,pwr)