Background extraction for multi-target spectrum.

[simontorres]

We are currently dealing with background extraction regions for multiple targets assuming they are fairly spaced, here is a list of the most likely situations I see:

• Targets too close to either edge of the image
• Targets too close to and edge and another target nearby
• Multiple targets in their conflicting their otherwise background extraction zone
• Blended sources, this is a different topic but relates to this particular problem as well

This issue should replace #13