Add feedback callback

[mkrogius]

Adds a callback for the feedback from the action server. The callback is called with the userdata and the feedback message

[lgeo3]

Hi, is this pullrequest going to be accepted ? (I need the userdata in the feedback callback, and this commit seems to provide it)

[mkrogius]

Its been over a year and I haven't heard anything back, so I think its safe to assume its not going to happen.

[bajo]

I fear the same, which is kind of sad, as this and some of the other open pull requests offer some nice additions so smach.
I for myself just cloned the executive_smach repo and merged the pull request into my local catkin workspace to be able to use them instead of the version that is released through the ros repository.

[lgeo3]

Ok, sad, @jbohren do you confirm ? Can we help you ?

[jbohren]

@laurent-george et al. The real issue is a lack of time for thorough testing.

I've been meaning to get around to not only that things for a long time (as you can tell by messages on this project pages), but also migrate SMACH to use coroutines instead of threads.