Add named frames to CollisionObjects

[felixvd]

This belongs to ros-planning/moveit#1060 and fixes #47 (now properly targets melodic, not kinetic).

[henningkayser]

@felixvd I really like this feature and hope #1060 will get merged soon!

[BryceStevenWilley]

LGTM, with one correction.

[rhaschke]

I generally approve this. Not sure yet, whether we really need the ValidateConstraintFrames service.
Shouldn't this be called TransformConstraintFrames, because it's main task is to transform the transforms into known link transforms.

[felixvd]

All of the alternatives I could come up with are somewhat imprecise and can be misunderstood. In my mind, the important point is that the constraints are changed from an unusable/invalid state to a formulation that the planner can use. I considered others, too:

• FixConstraintFrames can be misread as "making immobile or immutable"
• ResolveConstraintFrames as suggested by @henningkayser reads too much like a lookup or processing operation to me
• ValidateConstraintFrames does not make clear that "repaired" constraints are returned
• TransformConstraintFrames sounds generic, does not express the intention behind the action

I think naming and readibility is important, so I'm all for being verbose if it helps understanding. The function does not come up often anyway, so it won't clog any code. TransformConstraintFramesToRobotLink would be fine in my book.