Add sensor thresholds

[rkdarst]

High-frequency sensors should have some sort of threshold so that data is only collected when things change.

Purple Robot implements this. It can be seen here:

• https://github.com/cbitstech/Purple-Robot/blob/master/Purple%20Robot/src/edu/northwestern/cbits/purple_robot_manager/probes/builtin/AccelerometerProbe.java
• Search for "threshold" within that file.

High frequency sensors can be found here:

• https://github.com/denzilferreira/aware-client/blob/master/aware-core/src/main/java/com/aware/utils/WebserviceHelper.java#L332
• Search for the "highFrequencySensors" array
• Currently accelerometer, gyroscrope, barometer, gravity, light, linear_accelerometer, magnetometer, rotation, temperature
  • proximity perhaps should be added to this
• Focus first on : accelerometer, light, linear_accelerometer, rotation, and proximity

This needs to be configured by some variable.

• Current sensor variables are status_accelerometer and frequency_accelerometer (for example).
• Add new variable named threshold_accelerometer. This is analogous to the meaning of the Purple Robot accelerometer threshold.
• Purple Robot can be used as inspiration for the default thresholds for each sensor, they worked well for us.
• Default value should be the lowest non-zero choice. Anything greater than zero produced a reasonable amount of data for us.
• The variable does not currently need to be exposed by the upstream aware-server (we don't link to it).
  • The first implementation doesn't even need it configurable in the app. We can leave it as server-only configuration, since the default will be good for most people.

[rkdarst]

Added in 03d99b1 and merged upstream.