Add WORKSPACE_BUCKET

[ErinWeisbart]

Adds WORKSPACE_BUCKET to DCP configuration allowing image files to be read from a separate bucket than non-image files (e.g. pipeline, load_data.csv, etc.). Closes #160

Also adds another demo project that reads images off the Cell Painting Gallery but uses a private S3 bucket for non-image files, showing WORKSPACE_BUCKET use in action.