Sensor locations may transmit multiple sensor data types

[carlpulley]

Each sensor location (e.g. wrist, waist, etc.) may generate multiple types of sensor data (e.g. accelerometer data, rotational data, etc.). The exercise classification code does not currently take this into account (it incorrectly assumes that each sensor will transmit just one type of information).

SensorNet and SensorNetValue need to be generalised so that they can store multiple types of sensor data. The size of this list should remain constant (at least for the current exercising session - though it may vary between different sessions).

These changes will impact the UserExercisesClassifier and ExerciseModel actor receive methods (in particular, the require statements that they have). The logical query model should (hopefully?) be untouched by these proposed changes.