Note about temperature units.

Due to ROM restrictions, we had to change Celsius/Fahrenheit switching from a runtime to a compile-time adjustment. This solved a number of problems, including interpreting the user's settings; we can now simply convert once, from A/D counts to Celsius or Fahrenheit, and accept the user's numbers in the same scale. We tied 12/24 hour time representation to this as well, but that isn't necessary.