make rosmsg tutorial more like thrift tutorial

@lornat75 I added two things from your thrift tutorial to the ros tutorial:
  * Use of `yarp read`. Read and write will work now for YARP master (it seemed silly not to support classic yarp bottle serialization so I just went ahead and added it).
  * Use of `CMake`. Works just like thrift. (As a separate issue: It feels like we could make this friendlier, the "ALLOW_IDL_GENERATION" flag is a bit awkward).

Separately, I plan to document how to get from this point to communicating with ROS equivalent programs, and other issues we discussed.