generate a fixed background as interpolation between fixed points

[briantoby]

This from Martin Kunz: when one wants to use the autobackground, but also then do some manual of the fixed points, interpolation would allow one to use the fixed background points in way that does not require fitting a function.

Other ideas:

• one might want to have the option to fit the autobackground only over a limited region
• it would be nice to be able to set the point density
• The "interpolate" functions are effectively fixed point functions. The red dots could be placed at the locations from the background function and movement of points could then be reflected in the background coefficients