filename selection for each recording

[guihomework]

Would be nice to be able to provide a filename for each new recording start.

For backward compatibility, keep the start stop, using the provided filename at node start, but also add a new topic with a more advanced triggering. To remain compatible with RSB, maybe a string message could be send:
start or stop and add a filename in the string
start:filename
stop:filename

[guihomework]

Partially fixed, for ROS only now.
Using filename:start and filename:stop. This way with an empty filename for stop sending via rostopic command line does not complain on yaml variable being empty (it does if 'stop:' is given whereas it accepts ':stop')