RobotModel.data doesn't contain all necessary data

[beverlylytle]

Describe the bug Some data is missing from in RobotModel.data making the serialization of instances of this class faulty. In particular, if a RobotModel is loaded from a urdf that contains gazebo or other nonstandard tags, some information will be lost upon serialization followed by deserialization. This is exposed by attempting to write the reserialized RobotModel to URDF.

To Reproduce Steps to reproduce the behavior:

1. Run the following script:

import copy

from compas.files import URDF
from compas.robots import RobotModel
import compas_fab

filepath = compas_fab.get('universal_robot/ur_description/urdf/ur5.urdf')
model = RobotModel.from_urdf_file(filepath)
model_copy = RobotModel.from_data(model.data)
# model_copy = copy.deepcopy(model)
urdf = URDF.from_robot(model_copy)

(Though this script imports compas_fab, it is only for accessing a particular URDF. The relevant code lies in compas.)

Expected behavior This should result in the creation of a valid URDF instance, however an exception is thrown about a URDFGenericElement not having a tag. This is because the protected attribute _urdf_source (or the inferred attribute tag) is not included in data.

[tomvanmele]

is this still a thing?

[beverlylytle]

Yes.

[tomvanmele]

robots no longer in compas core...