Deaccumulation of ADC18 and Scaler channels need to be implemented for n = 0 and n = 1

[wdconinc]

Environment: (where does this bug occur, have you tried other environments)

• branch (master for latest released): feature-eventring-unwind
• revision (HEAD for most recent): HEAD
• OS or system: gcc8
• Special ROOT or Geant4 versions? no

Steps to reproduce: (give a step by step account of how to trigger the bug)

1. Run build/qwparity -r 1296 -e :10k --ring.print-after-unwind

Expected Result: (what do you expect when you execute the steps above)

1. The rolling average of the event ring after unwinding should be zero for all channels

Actual Result: (what do you get when you execute the steps above)

1. Non zero.

This should involve a similar fix to what is in the VQWK channel right now for n2 == -1 and n == 0 or n == 1. It should also include the rounding fix to prevent 0 from showing up as nan when the subtraction leads to a tiny negative epsilon.