Baseline Optimizer: Allow etalon selection

[nthallen]

The etalon analysis tool is awesome, but could benefit from a few more tweaks:

• No point in calculating/displaying etalons much narrower than the narrowest lines
  • This is already implemented by setting etalon_maxf in the object.
  • Constraining maxf increases the resolution of the etalon detection, perhaps beyond the point of relevance
• List multiple peaks from the analysis along with already added etalons and allow user to choose which to add to or subtract from the baseline.
  • Sometimes the analysis chooses a value already present, which is not likely to end well.
• Generate a function to recreate the baseline, allowing the user to tweak independently