* Add <D SERVO vpin position> command
Allow a PWM servo to be driven to any arbitrary position.
* Enhancements for HAL drivers
Add state change notification for external GPIO module drivers;
Allow drivers to be installed statically by declaration (as an alternative to the 'create' call).
* Create IO_HCSR04.h
HAL driver for HC-SR04 ultrasonic distance sensor (sonar).
* Enable servo commands in NO-HAL mode, but return error.
Avoid compile errors in RMFT.cpp when compiled with basic HAL by including the Turnout::createServo function as a stub that returns NULL.
* Update IO_HCSR04.h
Minor changes
* Change <D SERVO>
Give the <D SERVO> command an optional parameter of the profile. For example, <D SERVO 100 200 3> will slowly move the servo on pin 100 to PWM position corresponding to 200. If omitted, the servo will move immediately (no animation).
* IODevice (HAL) changes
1) Put new devices on the end of the chain instead of the beginning. This will give better performance for devices created first (ArduinoPins and extender GPIO devices, typically).
2) Remove unused functions.
* Update IO_HCSR04.h
Allow thresholds for ON and OFF to be separately configured at creation.
* Update IODevice.cpp
Fix compile error on IO_NO_HAL minimal HAL version.
* Update IO_PCA9685.cpp
Remove unnecessary duplicated call to min() function.
