parser_urdf should not ignore origin of floating and planar joints

[scpeters]

Desired behavior

While investigating a question related to the reference frame used for floating URDF joints, I noticed that parser_urdf.cc currently takes no action when encountering floating and planar joints (see the switch logic where no value for jtype is set and the subsequent if block). This causes the //joint/origin tag to be ignored for these joint types, when it should be passed along to the SDFormat file in the link pose.

Alternatives considered

Implementation suggestion

Additional context

[scpeters]

A first step towards fixing this would be to add a URDF file with a floating joint that has a non-zero origin to the test/integration folder.

A second step would be to write a test that parses this file and expects the child link of the floating joint to have a pose that matches the value from the floating joint origin.

A third step would be to update parser_urdf.cc to translate the floating joint origin to the child link, perhaps by adding a frame for the floating joint and defining the child link pose relative to that frame.