Improve serial device for Dextra hand

[PeterBowman]

Closes #176. Implements the controlboardwrapper2 subdevice that interfaces with the Teensy via USB serial port.

• the YARP device has been renamed from DextraControlboardUSB to DextraSerialControlboard
• instantiates YARP's serialport device under the hood to take care of comms
  • pros: no need to meddle with low-level details, multiplatform (no longer requires a UNIX system)
  • cons: requires ACE (recompile YARP with -DSKIP_ACE:BOOL=OFF)
• Teensy-compatible protocol ensured via C++ port of synapse.py (see also firmware side)
  • serial port configuration defaults picked from pySerial implementation
• unused interface implementations have been removed: torque control, velocity control (this one is required by the YARP wrapper, therefore I left it, but all methods return false), interaction mode
• I'd rather nuke position control as well and move motion commands to the position direct implementation, leaving a few methods such as positionMove solely for ease of use via terminal+RPC; currently starts at position control mode, though, since yarpmotorgui does not let me enable the desired position sliders in position direct mode (future work)
• implements IAxisInfo, which labels joints accordingly (see capture below)
• yes, now you can connect to the RPC port and set pos any Dextra's joint as expected, even via yarpmotorgui (can be started with all robot parts + both dextras, thus allowing to move robot + hands within the same GUI)

[coveralls]

Coverage remained the same at ?% when pulling 14a9c8d381d574188929a5aee8d0ec2e1f8b10c9 on fix-176-dextra into fc417a504e1143f5f65e3e4d36027105fc2021a6 on develop.