Adding RapidJSON as a third party module

Since ADIS uses RapidJSON but VTK uses JSON Cpp, we've decided to pull the RapidJSON source into ADIS as a third party module.

Also something to be aware of, for VTK and VTK-m, the robot ensures that no one makes edits to the third party directory (outside of running the update script). However we don't have a robot for ADIS to do this.