-Changed SensorReading duplicate constructor to call the receiving
SensorReading instance's constructor;
-Removed SensorReading's clone constructor (silly me, you can't
construct an abstract class directly),
-Pulled the various SensorReading sub-types' formatted timestamp Strings
up into SensorReading, and refactored code to set its value
appropriately;
-Refactored ReadingCircularBuffer code to receive SensorReadings instead
of AccelReadings;
-Minor refactoring;