Allow name attribute in visual tag

[gerkey]

With some URDF like this:

    <visual name="tire">
      <origin xyz="0 0.06 0" rpy="${M_PI/2} 0 0" />
      <geometry>
        <mesh filename="${mesh_dir}rp1a_wheel.dae" />
      </geometry>
      <material name="MidGrey"/>
    </visual>

I get a warning like this:

Unknown attribute "name" in /robot[@name='resource_prospector']/link[@name='front_right_wheel_link']/visual[1]

According to the docs, it's valid to have a name attribute in a visual tag:
http://wiki.ros.org/urdf/XML/link#Elements

In this PR I made changes that I think are in the right direction to add support for that attribute. At least, with these changes, the warning disappears. But I must admit that I'm not certain that I've done the right thing here.

It might also be a good idea to update the schema: https://github.com/ros/urdfdom/blob/master/xsd/urdf.xsd#L123-L133, but I'm not sure whether that's necessary.

[ri-ceres]

Is there any reason why this shouldn't be merged? The missing name attribute breaks using the parser to find elements by name in Melodic, which previously worked in Kinetic.

[clalancette]

Is there any reason why this shouldn't be merged? The missing name attribute breaks using the parser to find elements by name in Melodic, which previously worked in Kinetic.

There are a couple of things here:

1. As-is, this patch breaks the API by adding an attribute in the middle of the argument list of the Visual constructor. This can be easily rectified by moving that to the end, however.
2. I don't see how this worked in Kinetic. The code around here is very similar, and we definitely didn't remove the 'name' attribute between Kinetic and Melodic, so I don't see how this could be a regression.

@ri-ceres Any thoughts on how this might have worked in Kinetic?

[sloretz]

As-is, this patch breaks the API by adding an attribute in the middle of the argument list of the Visual constructor. This can be easily rectified by moving that to the end, however.

Moved to end in 624bc73