Add test of two Gaussian model

* fit_utils.py: Create a fake two-gaussian model and save it to a fits
  file
First complete version of nebular fit

It still has issues.

* fit-nebula.py: We fit polynomials to the 2-delta decomposition in the
  BG region, and then use these to initialise Gaussian components for
  the full 2D fit.  The problem is that the fit does not guarantee that
  the width remais positive, so we need to fix that.