New features for visualizer, and issue

[cmower]

The new visualizer is intended for debugging urdf's, and visualizing robot states. Currently, its use is fairly limited. This is some features that would be good to implement, and also a bug that needs fixing.

• [x] [bug] Visualize links. Currently I believe there is a bug in the following method. When I try to visualize the robot links, the center of the link is correctly positioned, but the cylinders that make up the axes are all in the wrong place. Update: I will just use lines for the axes rather than cylinders. See 617e216f019b728e1c6ea809b3270e56f2a67bad.

https://github.com/cmower/optas/blob/ee6e02f6adbcb75c42785f303b0c93daeda35913/optas/visualize.py#L217-L252

• [x] [feature] Support materials from the urdf. Currently, links are visualized in white. The colors and potentially textures should be supported.
• [x] [feature] Visualize additional objects (e.g. boxes/spheres).
• [x] [feature] Visualize robot motion, i.e. a planned trajectory. Option: video-link motion, and also a static figure where older states decrease the opacity of the robot.
• [x] [feature] Support loading several robots at once.
• [x] [feature] Support visualization of task models/plans (e.g. a 3D trajectory represented by a sphere flying through space)
• [x] [feature] Support visualization of other meshes (e.g. tables)
• [x] [feature] The above may need a revamp of the RobotVisualizer class, and also the visualization.py example script will need to be updated

[cmower]

Closing, once #88 is merged this is resolved.