RobotModel.data doesn't contain all necessary data

[gonzalocasas]

Migrated from compas-dev/compas#730

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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.

[yck011522]

@gonzalocasas is this still relevant? What is the use case for compas_fab to read a URDF with "nonstandard tags"?