Load C++ Messages from YAML

[threeal]

As in #527, there's already a to_yaml() function that could be used to save messages as a YAML file. Would there be some kind of from_yaml() function that reverse the action of the to_yaml() function so it could be used to load messages from a YAML file?.

[clalancette]

Please open questions like this on https://answers.ros.org, which is our Q&A site. That way it is one searchable place.

I'm going to close this out for now, but feel free to reopen if you feel this is in error.