[k] = wavevector(obj, vWavelength, fTheta, fPhi)
 [fOrdinaryWavevector, fExtraordinaryWavevector] = wavevector(obj, vWavelength, fTheta, fPhi)
 [fSlowWavevector, fFastWavevector] = wavevector(obj, vWavelength, fTheta, fPhi)
    2015.06.28 revision