Return the tonnetz projection matrix.

def _make_tonnetz_matrix():
    """Return the tonnetz projection matrix."""
    pi = np.pi
    chroma = np.arange(12)

    # Define each row of the transform matrix
    fifth_x = r_fifth*(np.sin((7*pi/6) * chroma))
    fifth_y = r_fifth*(np.cos((7*pi/6) * chroma))
    minor_third_x = r_minor_thirds*(np.sin(3*pi/2 * chroma))
    minor_third_y = r_minor_thirds*(np.cos(3*pi/2 * chroma))
    major_third_x = r_major_thirds*(np.sin(2*pi/3 * chroma))
    major_third_y = r_major_thirds*(np.cos(2*pi/3 * chroma))

    # Return the tonnetz matrix
    return np.vstack((fifth_x, fifth_y,
                      minor_third_x, minor_third_y,
                      major_third_x, major_third_y))