includes the following changes
move WINCH_ENABLED to library
add singleton
add pre_arm_check
backend gets init_input_and_output and read_pilot_desired_rate
remove use of wheel encoder
rename servo-with-encoder to just servo
add write_log
rename control_mode
add rc input processing
add acceleration limiting
fixes after peer review:
renamed disable to relaxed
make parameters private
default type to 1
enabled method checks backend created
servo range initialised in init method
contrain rate-desired during position control
use set-output-limit instead of set-safety-limit
release_length accepts rate
Randy Mackay
Peter Barker