vESD = energySpectralDensity() - assuming pulse is in vacuum
 vESD = energySpectralDensity(fMaterialRefractiveIndex) -
 vESD = energySpectralDensity(sAxisType) -
 vESD = energySpectralDensity(fMaterialRefractiveIndex, sAxisType) -
 
 1D-T (XY Gauss assumed)
 2D-TX (Y Gauss assumed)
  \int dx \int dy n/2/Z0 |E(\omega)|^2 [J/Hz]
 
  fMaterialRefractiveIndex refractive index of current material
     (default 1)
  sAxisType 'AngularFrequency' (default) [J/Hz], 'Frequency' [J/Hz],
     'Wavelength' [J/m]