rise on each hit:

   dy = (1 - y) / kRise

 fall after each time unit:

   dy = y / kFall

 fall after time dt:

   dy = - y ** - (dt / kFall)

 after the next hit:

   y = y - y ** (- dt / kFall) + (1 - y) / kRise

first attempt at a load measurement algorithm:
  - kFall is an arbitrary constant which dictates decay rate of load
    in the absence of hits
  - kRise is another constant which dictates rise of load with each hit
  - dt is the time between each hit