Return the analytical potential in distance r over a homogeneous
    half-space

def pot_ana(r, rho):
    """Return the analytical potential in distance r over a homogeneous
    half-space
    """
    I = 1.0
    sigma = 1.0 / rho
    phi = np.divide(I, (2.0 * np.pi * sigma * r))
    return phi