JTC trajectory end time validation fix

[henrygerardmoore]

The time_from_start member for a JointTrajectoryPoint is monotonically increasing, not a delta time, so I think we can just use the last one in the vector; summing them up seems incorrect. I also moved the points empty check earlier because .back().time_from_start would seg fault with an empty points vector.

[christophfroehlich]

Could you please check the docs if this is clearly documented how to fill the trajectory field? Here is a comment, but maybe we should add this in a more prominent place.

[henrygerardmoore]

Could you please check the docs if this is clearly documented how to fill the trajectory field? Here is a comment, but maybe we should add this in a more prominent place.

@christophfroehlich That page seems clear to me, the plots in there are very helpful. Perhaps it should be in the main JTC page, but where it is seems fine IMO.

[henrygerardmoore]

@christophfroehlich is there anything I can do to help get this across the finish line?

[christophfroehlich]

No, we have to wait for an approval of @destogl or @bmagyar