Changed to Set CellBlender Project Directory

Changed the Export CellBlender Project to Set CellBlender Project Directory
Added intro.initialization.mdl file

Added intro.initialization.mdl file, but I'm not sure if the viz and rxn files should be moved. When I run CellBlender, those files are after the INSTANTIATE block, but they're listed before it in the example.
Merge https://github.com/cnlbob/cellblender_tutorials

Conflicts:
	getting_started.rst