Calculate Wigner distribution

W = mywigner(Ex)

W = output Wigner distribution
Ex = Input electric field (MUST be a column vector

Notes:
W = Int(-inf..inf){E(x+y)E(x-y)exp[2ixy]}
E(x+y) & E(x-y) are calculated via a FFT (fast Fourier transform) using the
shift theorem. The integration is performed via a FFT. Thus it is important for the data to satisfy the sampling theorem:
dy = 2*pi/X X = span of all x-values dy = y resolution
dx = 2*pi/Y Y = span of all y-values dx = x resolution
The data must be completely contained within the range x(0)..x(N-1) & y(0)..y(N-1) (i.e. the function must fall to zero within this range).