Add support for plant sensors

A good plant health sensor would have:

• the moisture percentage in the ground
• the conductivity of the soil
• the lumen level falling on the plant
• the temperature of the soil
• the temperature of the air

[benfrancis]

• the temperature of the soil
• the temperature of the air

These could both use TemperatureProperty.

• the lumen level falling on the plant

We just added a ColorSensor capability, but I guess this is different. We do have a BrightnessProperty but that's a percentage. Maybe something like a LuminanceProperty with a unit of lumen?

• the moisture percentage in the ground
• the conductivity of the soil

These are definitely new. Are there SI units for moisture and conductivity?

Not sure whether this should be a PlantSensor capability or separate TemperatureSensor, MoistureSensor and LightSensor. Or maybe we could do both.

Maybe something like a LuminanceProperty with a unit of lumen?

I believe that would be correct.

Moisture would be a percentage.

I'm not sure about conductivity.

Here is a professional device:
https://www.metergroup.com/environment/articles/meter-legacy-soil-moisture-sensors/

Their website has many details:
https://www.metergroup.com/environment/articles/soil-electrical-conductivity-beginners-guide-measurements/

It mentions:

The SI unit for electrical conductance is the Siemen, so electrical conductivity has units of S/m.

I don't think a capability is required, unless that makes it easier to create advanced displays.

[mrstegeman]

The UI builds displays based on the exposed capabilities. That goes for things, properties, actions, and events.