Fix bug possible erroneous HN24 values bug

In certain cases, HN24 can be reported erroneously. What triggers this
is e.g. a storm which deposits 5cm of low-density snow which quickly
settles. During such a storm, the HS over a 24-hour period may go from
e.g. 100cm to 105cm then back to 100cm.

The root cause was taking the difference between the highest HS value
reported over a 24-hour period and subtracting the lowest value. But
when HS fluctuates throughout a storm due to rapid settlement and/or
wind, this calculation does not provide correct data.

Additionally, this bugfix opens the door to reporting HS settlement to
InfoEx should InfoEx support that data via autostations.