[ASK NICK] throw useful exception about missing thickness field
create operation to filter data
shape comparison must be against selected background
improve handling of options for subtraction

 * needed to add a few more methods to provide information required
   to figure out how to handle the background data
ensure that the laziness of the BG dataset is honored for longer
check shapes better, fix number of images method
move some code around, rename total to sampleImages
fix comparison of dataset shapes

 * get slice of background data