[Spinal][Dynamixel] create a interface to control dynamixel with Spinal

[sugihara-16]

What is this

Workaround to support direct Dynamixel control from Spinal.

Details

The followings are supported:

• Sending and receiving commands via rostopic (e.g. send goal position 2700 to ID 1 servo)
  $ rostopic pub -1 /servo/target_states spinal/ServoControlCmd "index : [1] angles: [2700]"
• Setting servo configuration via rosservice (e.g. set gains p = 2500, d = 500, i = 10000 to ID 1 servo)
  $ rosserive call /direct_servo_config spinal/SetDirectServoConfig “command: 2 data: [1, 2500, 500, 10000]”

TODO

• make a common servo interface that works in both Spinal and Neuron
• implimente gui configurator for direct control servos
• separate the current servo interface into TX/RX-related part and the other