Use "persistence" option from yarp.Network.connect to create persistent connections

[nunoguedelha]

Preliminary checks

• "persistent" option working on the terminal.

  yarp connect --persist OUTPUT_PORT INPUT_PORT
  yarp connect --persist-from OUTPUT_PORT INPUT_PORT
  yarp connect --persist-to OUTPUT_PORT INPUT_PORT
  

• Test in Javascript/Node.js : ⚠️ FAIL ⚠️ . Bindings to the class yarp::os::ContactStyle are not implemented.
  ...

Other tests and obsevations:

...

• Creating a persistent connection before creating the /write /read ports only works if we use the prefix topic://aTopic. Doing:

  yarp topic /aTopic
  yarp connect /write /aTopic
  [ERROR] |yarp.os.Network| Failure: could not find source port /write
  

  doesn't work without the --persit explicit option.
  ...

Conclusion

Having this in mind and the fact that Bindings to the class yarp::os::ContactStyle are not implemented, it seems easier to use topics for creating persistent connections, although the approach using the style option would be simpler if yarp::os::ContactStyle was available.

The drawback is that we won't be able to cleanup the topics once we don't need them anymore (unless we run a child process just for that of course, which we should avoid).

It might be worth trying to create the bindings for the class yarp::os::ContactStyle. If it's not straightforward enough, we'll go for the topics solutions.

Originally posted by @nunoguedelha in #68 (comment)

Depends on robotology/yarp.js#26.