Hello World with Raja

Demonstrates basic interoperability with Raja, using Charm++ nodegroup
to spawn a separate instance of Raja on each process.

Requires Raja to be built with GPU support.
e.g. From Raja source folder,
> mkdir build && install
> cd build
> cmake -DENABLE_CUDA=On -DCMAKE_INSTALL_PREFIX=<path to RAJA install folder> ../
> make -j
> make install

Path to Raja install folder and GPU compute capability should be set in Makefile.common.
