Add hysterisis to warnings (e.g. over temp)

Add the ability to add hysterisis for warnings such as temperature, disk space etc.

Lets say you have your disk temp threshold for a warning at 45C, when the drive gets close, it might vary slightly -> 44>45>44>45>44>45 etc. Each time it moves from 44 to 45, a warning message (for any notification methods chosen, e.g. email), which seems excessive. If some hysterisis was added, for example to be able to say "don't re-alert unless it drops under 43C", then you'd only get the notification on the first instance, then you wouldn't get further notifications.

This could be user configurable, so you can make it 1C (current behaviour) if desired, but allows the reduction in notifications if desired.

Same idea for disk space (e.g. require a 5% drop before re-notification, so the removal/addition of a small file doesn't re-trigger (e.g. a cache drive that might be continually changing). There may be other scenarios too, but they're the only ones that come to mind.