fix number of images used for calculations

 * after filtering, the number may be different from counter
calculate error correctly

 * should take square root, then divide
allow user to choose to use filtering
use different parameters for actual Rg-based filtering
add errors to Rg, fix filter to keep items that are false
output filtering data if used

 * useful to know which frames have been rejected and the Rg values
 * do not forget to name datasets!!!
move slicer