Support metric units from the controller

[SphtKr]

The aqualogic API has a method to detect this condition, but it's unclear when its result will be valid (and in theory, it could change!) and in any case it will happen after the discovery metadata is published. Likely, the values will probably need to be always published to MQTT in one set of units, and be converted by aqualogic_mqtt if necessary if they don't match what is published, unfortunately.

One less elegant option would be to require the user to specify as an argument whether metric units are expected from the controller, and make it the user's responsibility to make sure it matches reality (else behavior is undefined). This may be the MVP solution, actually.