add test to build robot model from python urdf classes

[k-okada]

add test to check building robot model using from urdf_parser_py.urdf.Robot class, for example

        robot = urdf.Robot(name = 'test')
        link = urdf.Link(name = 'link',
                         visual = urdf.Visual(geometry = urdf.Cylinder(length = 1, radius = 1),
                                              material = urdf.Material(name = 'mat')))
        robot.add_link(link)

[k-okada]

Failing http://build.ros.org/job/Mpr__urdfdom_py__ubuntu_bionic_amd64/11/console is expected, because eaea61f cause regressio on creating robot model with visual and collisions.
When we output URDF from the robot model built with #45 (comment) , it misses visual and collision tags like

<robot name="test">
   <link name="link"/>
</robot>

13:47:31 ======================================================================
13:47:31 FAIL: test_link_material_missing_color_and_texture (test_urdf.TestURDFParser)
13:47:31 ----------------------------------------------------------------------
13:47:31 Traceback (most recent call last):
13:47:31   File "/tmp/ws/src/urdfdom_py/test/test_urdf.py", line 188, in test_link_material_missing_color_and_texture
13:47:31     self.xml_and_compare(robot, xml)
13:47:31   File "/tmp/ws/src/urdfdom_py/test/test_urdf.py", line 38, in xml_and_compare
13:47:31     self.assertTrue(xml_matches(robot_xml, orig_xml))
13:47:31 AssertionError: False is not true
13:47:31 -------------------- >> begin captured stdout << ---------------------
13:47:31 Match failed:
13:47:31 <robot name="test">
13:47:31   <link name="link"/>
13:47:31 </robot>()
13:47:31 ==============================================================================
13:47:31 <robot name="test">
13:47:31   <link name="link">
13:47:31     <visual>
13:47:31       <geometry>
13:47:31         <cylinder length="1" radius="1"/>
13:47:31       </geometry>
13:47:31       <material name="mat"/>
13:47:31     </visual>
13:47:31   </link>
13:47:31 </robot>()

[sloretz]

It looks these changes are also in #47, so I'll close this.