Modelled gyroscope as Sensor

Modelled gyroscope as Sensor which enable the calibration.
The reference vector of Gyroscope is the Bias.
Two-sensor model mode added

Lock Bias mode added
Two-sensor calibrator added
GUI changed for adaption
Merge pull request #3 from minhcly/sensor_reduce

Sensor reduce branch merged.
Added Two-sensor model mode, allowed users to use only 2 sensors: gyroscope and accelerometer.
Important changes:
+ Gyroscope has benn modeled as Sensor, allows it to use the calibrating function. The reference vector will be the static bias.
+ Lock bias mode added in Filter, Gyroscope. Lock bias mode locks the bias to a static value, which will not be updated in Kalman update phase, thus increases the stability.
+ TwoSensorModelCalibrator added, abstracted Calibrator class. Old calibrator class changed into ThreeSensorModelCalibrator class.
+ GUI changed for adaptation.