Optimize files created for URDF

[openvmp]

The STL files created in <openvmp_root>/platform/src/openvmp_robot_don1/meshes for URDF look ugly.

Perhaps the STL export parameters should be tweaked.
Alternatively some post-processing is necessary.
Either something like "optimize STL for 3D printing" can be done, or another URDF-supported mesh file format (also supported by both Gazebo and Rviz) should be used.