Fix planar joint Jacobian bug

[ivatavuk]

Description

While working on this I think I found a bug in Jacobian calculation for planar joints.

The fix is simple, in lines performing joint_axis calculation, instead of joint_transform, joint_transform.linear() should be used, like for the other joint types.

I've written a simple test, I don't think it needs to be merged, but it demonstrates the bug so you can check it.
For the example in the test, current getJacobian implementation calculates the joint_axis of the first joint to be [1, 0, 0]^T when q[0] = 0, and [11, 0, 0]^T when q[0] = 10, which results in different Jacobian values.

[welcome]

Thanks for helping in improving MoveIt and open source robotics!

[codecov]

Codecov Report

Patch coverage: 64.04% and no project coverage change.

Comparison is base (ba04810) 61.82% compared to head (9475dfb) 61.81%.

Additional details and impacted files

@@               Coverage Diff                @@
##           noetic-devel    #3439      +/-   ##
================================================
- Coverage         61.82%   61.81%   -0.00%     
================================================
  Files               380      385       +5     
  Lines             33610    34064     +454     
================================================
+ Hits              20775    21054     +279     
- Misses            12835    13010     +175     

Impacted Files	Coverage Δ
...lude/moveit/collision_detection/collision_matrix.h	100.00% <ø> (ø)
...bot_model/include/moveit/robot_model/joint_model.h	89.34% <ø> (ø)
...bot_model/include/moveit/robot_model/robot_model.h	84.00% <ø> (ø)
...bot_state/include/moveit/robot_state/robot_state.h	85.12% <ø> (+0.76%)	⬆️
moveit_core/utils/src/xmlrpc_casts.cpp	0.00% <0.00%> (ø)
...moveit/move_group_interface/move_group_interface.h	36.85% <ø> (ø)
.../moveit/setup_assistant/tools/moveit_config_data.h	44.45% <ø> (ø)
...tup_assistant/src/tools/collision_linear_model.cpp	0.00% <0.00%> (ø)
...t_setup_assistant/src/tools/moveit_config_data.cpp	14.34% <0.00%> (-0.76%)	⬇️
moveit_core/robot_model/src/joint_model.cpp	52.76% <12.50%> (-2.70%)	⬇️
... and 14 more

... and 9 files with indirect coverage changes

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[rhaschke]

Thanks for this fix.

[welcome]

Congrats on getting your first MoveIt pull request merged and improving open source robotics!