Simplify and fix set target temperature calculations

[mib1185]

(plain - 16) / 2 + 8 - this subtracts 16 from the value, halves it and then adds the half of of 16 again - so just halve the value is the same

int(16 + ((float(temperature) - 8) * 2)) - this subtracts 8 from the value, doubles it and add the double of 8 again - so just double the value is the same

temp > max(range(16, 56)): this was already true for temp=56 instead of a temp higher than 56

[coveralls]

coverage: 96.856%. remained the same
when pulling 067d1a5 on mib1185:simplify-target_temp-calculations-and-fix-set-max-target-temp
into 39d7510 on hthiery:master.