Adjust activity decoders so that no activity maps to 0% activity

[terrillmoore]

Right now, no activity maps to 0.02% activity when the data is passed through the system. This is an artifact of the various scaling conversions. The filter in the 4430 software will settle at a final value (-1) after a short period of inactivity; but for whatever reason, this is transmitted as a value that isn't quite full scale [-1, 1]; subsequent mapping onto [0,100%] is not quite mapping minimum to zero. Probably also not mapping 1 to 100%, but that's not such an observable thing.

Add unit tests that will exercise this, as part of correcting the code.

Since we have many formats, this probably requires searching through all the decoders and test vectors and correcting.