Add 2-component decomposition of profile

* fit-nebula.py:  Calculate the positions (v1, v2) and intensities (I1,
  I2) of the two delta functions that have the same (intensity, mean,
  sigma, skewness) as the observed line.
Improve the moment plots

* plot_utils.py:  Additional lines showing the intensities and
  velocities of the 2-delta decompositions of the line profile.