Minor modifications to stamp extraction

* extract-stamps.py: Zero point of y axis is now always at center of
  order
New routines to fit and remove the continuum

* remove_continuum.py: This does it for a single line

* remove_continuum_all.py: And thi does it for all of them
Make a start at dealing with the doublets

* doublet_utils.py:  Functions to find equivalent doublets and do the
  deconvolution