Use Grasp msg in Pick / Place stages

[rhaschke]

Pick / Place as well as SimpleGrasp / SimpleUnGrasp stages should be modified to use Grasp msg. Currently, we have only GenerateGraspPose, which directly operates on named joint states. However, extending this to support full Grasp msg will facilitate replacing it with more elaborate grasp providers.

We should also rethink Grasp msg: Its fields pre_grasp_posture and grasp_posture are trajectories, which is neither reflected in the name nor in currently used semantics. I suggest to merge them into grasp_trajectory, which can be any finger trajectory. The current use case is implemented, adding pre_grasp_posture as first and grasp_posture as last waypoint of the trajectory. Additionally, it would be useful to define an ungrasp_trajectory.

[v4hn]

Related to https://github.com/ros-planning/moveit_task_constructor/issues/112