The dynamic import should now work when MAT2 is
installed system-wide, either via the distribution's
packaging system, or via pip.

- the `src` folder is now `libmat2`
- the `main.py` script is now `mat2.py`

- Simplify the multiprocessing by using a Pool
- Use some functional () constructions to exit
  with a return code
- Add some tests to prove that we're doing things
  that are working correctly