Added a check for the transmission state in the dynamixel communication module...

Added a check for the transmission state in the dynamixel communication module before starting a new transmission.
Initialized the motion_manager module before calling the user_init() function. This way the dynamixel communications are already set up when the user function is called.
The initialization of the expansion baord is attempted by default. If the board is not found, it will not be used.
Added the expansion board loop function in the motion manager loop for timing.
Added a preprocessor command to either read all expansion board data in the loop() function or read each filed using a separate function.