Should provide additional conversions

[EricCousineau-TRI]

Follow-up to #153.

This is primarily in service of ROS 2 development in Anzu. Integration with Drake is a close second, though. We should either provide these here, or as a package in this workspace.

We should provide both C++ and Python conversions, and exercise them.

• [x] Python versions of geometry_conversions.h (either bindings, or reimplementations)
• [ ] Eigen matrices / NumPy arrays
• [ ] Point Clouds (\cc @hongkai-dai)
  • [ ] This could be Eigen matrices, and/or Open3D types.
• [ ] Images

\cc @IanTheEngineer @sloretz