Composable robot interface

[PetervDooren]

Goal:
Different robots have different capabilities. We have noticed that a robot interface should not be designed for just one robot but rather should be able to describe many different robots. We want to create a robot interface that can be composed of multiple different parts.

[PetervDooren]

Wisdom from @jlunenburg:
Basically, we want a couple of things:

• i) small, composable classes (way more flexible, not every robot is the same),
• ii) robot (not the smach state) to be aware of its structure) so that it knows which gripper is attached to which arm and
• iii) nice ' getter' methods so that the smach state does not need to know about the structure of the robot

Best to tackle not too many things at a time. Currently, Peter is working on i) (and ii)

This would result in robot.arm.gripper.send_goal(open) which is i) not generic because it makes assumptions on the structure of the robot and ii) breaks the api so that robot smach states do not work anymore

to resolve ii, we can temporarily add def send_gripper_goal(goal): return self.gripper.send_goal(goal)
This way, we can merge everything without breaking stuff. Next up, we fix iii