incorporated TF scatter approach into KWL section

Tried to bridge explanation of axes shifting into TFR approach in order to clarify use of slit spectra and to estimate the S/N that can be achieved. Some better quantification of scatter and perhaps using only sigma_gamma or sigma_epsilon might help (does sigmaTF = 0.05 -> sigma_epsilon = 0.015? Why use 0.05 instead of 0.056 or 0.058 from Miller or Reyes?)