IMU and magnetometer driver update and covariance customization

[hippo5329]

Recently, I updated the IMU and magnetometer drivers.

1. The MPU6050 driver will do the calibration during initialization. The MPU9150 driver is removed and uses MPU6050 driver.
2. The reading of magnetometers is scaled to Tesla.
3. The covariance matrix of the sensors, which is important to robot state estimation, can be fine tuned in the configuration file.
4. The pwm driver support multiple PWM freqency for esp32 now. PWM_FREQENCY and PWM_BITS will be used for pwm motors drive, while the SERVO_FREQ and SERVO_BITS will be used for ESC drive and servo motors.

Have fun.

• Thomas