Create a remove_background_signal function

[jordiferrero]

Describe the functionality you would like to see.

A method called remove_background_signal where a Signal1D object is subtracted from another Signal1D object.
If the axes don't match (in size, offset and scale for UniformDataAxis), the background signal should be rebinned/interpolated to the main signal axis.

Describe the context

When trying to fix some warning bugs, discussion made us realise of the need to rewrite the depreciated remove_background_from_file for a more HyperSpy like method, as discussed here #114 .

Additional information

Ideally it should also support non-uniform data axes with interpolation.
Maybe add support a for fitting a ScalableFixedPattern as @ericpre proposed.

Things to do

• Decide on where to add this method: Should it be done here in LumiSpy or in HyperSpy Signal1D class?
• Decide on the approach to take: Should we use rebinning/interpolation or ScalableFixedPattern?
• Write the method for UniformDataAxis
• Expand the method to also work for non-uniform axes.

[jlaehne]

For the method to work with non-uniform axes, we will first have to implement rebinning for non-uniform axes, which would be helpful in any case.