Launch file dependencies for tasks

[hawesie]

Some tasks require nodes to be running that

a) don't need to be running all the time
b) use up significant resources
c) take a noticeable time to start up

(vision, we're looking at you here)

The proposal is to allow the task executor to bring up and tear down a task's dependencies on demand. This is discussed in https://github.com/strands-project/strands_management/issues/52 and https://github.com/strands-project/strands_management/issues/66.

We will

• add an action server which will bring up and tear down nodes via roslaunch
• this action server should be told which services/topics to wait for before returning
• get the task executor to launch this either before moving to the task, whilst moving to the task, or after moving, starting with whilst as this is probably the main use case.

Depending on scheduler development we can also schedule bring-up/tear-down as tasks, but this relies on much of the above infrastructure anyway.

[hawesie]

roslaunch API, e.g. http://docs.ros.org/groovy/api/roslaunch/html/roslaunch-pysrc.html

[hawesie]

Tonight @Jailander volunteered to do this. Please can you do #20 too?