Users of fahrenheit generally expect to see whole degrees. The fahreneit
scale is suffiently precise that decimals aren't really useful in terms
of temperatures humans care about.
This change rounds fahrenheit values to whole degrees and celsius values
to one decimal place. It also renames the ThermostatDevice._convert()
method to _convert_for_display(), making its purpose more clear. It is
not useful for the min_temp() and max_temp() methods, as those relate to
the internal state of the ThermostatDevice object, and may use different
units. Adding optional source and target units to _convert() would have
added needless complexity, it's cleaner to just use convert() in those
methods.
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