RobotModel.data doesn't contain all necessary data

Migrated from https://github.com/compas-dev/compas/issues/730

---

**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`.