add initial background subtraction operation and model
implement background subtraction, assume 2-dimensional data (frames)
remove TODO - this is the same method as used in NBSForkJoinTransformer
set background slice based on Jake's method

 * Jake's code in class SubtractionBackgroupImageSubtraction (sic?)
 * also add comments about usage - this operation is to be used with
   images
Merge branch 'SCI-2041'