[vD1,vD2, vDelta] = getPolarizationDirections( vWavelengths, fTheta, fPhi)
    gives direction of principal polarization axis.
    vD1 - for e1 = e which is slow in Positive NLO frame (fast
    in negative NLO frame)
    vD2 - for e1 = e which is fast in Positive NLO frame (slow
    in negative NLO frame)
    2014.03.14