Transforms phasor u to amplitudes {alpha, beta}.
i.e. rotates phasor in polar representation by angle theta and transforms to Euclidean {y[1], y[2]} coordinates.
Input u:
u[1] absolute value u[2] argument, phase
Output y:
y = sqrt(2/3)*u[1]*{cos(u[2] + theta), sin(u[2] + theta)} amplitudes alpha, beta