`NoiseReplacerTask` wants some statistics about the background that was subtracted from the exposure, but it gets these in a fragile and roundabout fashion: it expects the code that measures the background to put the mean and variance into the exposure's metadata, using special keys. It is difficult to enforce correctness because background is measured several times while processing an exposure.
To solve this, pass the background directly to `NoiseReplacerTask`. This will require passing the background through the various measurement tasks, which will require small changes to code that calls the measurement tasks.
In addition, remove computation of background statistics from the background fitting code.