Adding:
- Python wrapper for the coupling framework.
  - Also, some utility methods.
  - One example of using the framework (simple advection).
- `source/Block.h` - `Block<...>` class, used by the Python wrapper.
- `applications` folder containing various utility methods.
  - P2M.h - Linear P2M algorithm.
  - Process.h - `process_pointwise` and `process_stencil`