add invert feature to Measure

[walter]

1) Invert the curve of the reading of sensor: Some of the resistive sensors
are Negative Temperature Coefficient (NTC), which means that the value
decrease in resistance when subjected to an increase in temperature,
humidity, etc. If teachers are working with young children, the value
may not be as important as the effect of the value rising at higher temp,
humidity, etc.

[walter]

added to Measure-27