Automatically installs robot models in the robots directory

[mmurooka]

The change in this PR will allow robot models in the robots directory to be automatically discovered and installed. This eliminates the need to make local changes to CMakeLists.txt each time a new robot is added.
For jvrc_mj_description, the behavior is the same as before; i.e., print FATAL_ERROR if not found, and call add_subdirectory if found.

[rohanpsingh]

Thanks! 👍