Implement background subtraction in create_meta_pixels

[paolomassa]

I think that it is necessary to implement background subtraction at the level of pixel counts. In my opinion, the best place to have it implemented is inside create_meta_pixels. We could have add a keyword bkg_cpd containing a compressed pixel data structure for a bkg measurement. If we do not pass bkg_cpd as input, then we do not perfom background subtraction

[samaloney]

Agreed we absolutely need to facilitate this but I'm not sure if create_meta_pixels is the best place.