endeffectors without associated groups could add implicit hidden groups

[v4hn]

At the moment defining an end_effector in the srdf file requires a group attribute to point to a valid joint_model_group. One of the main purposes of the end_effector structure these days is to decide where to show interactive markers in the motion planning display.

It is quite annoying to be forced to define new joint_model_groups with no active DOF just to define an end_effector at the tip of some structure you want to solve IK for. Thus it would be very convenient if

1. adding a <end_effector> tag with no group attribute would simply create a trivial hidden JointModelGroup in the background that consists only of the parent_link.
2. It would be impossible to select JointModelGroups with no active (or no non-fixed) DOF in the motion planning display (or construct a MoveGroupInterface for.