Feature/multiprocessing

For faster matrix generation
Now that the matrix is a proper object in itself, I was contemplating overriding `__str__` for this (and `__repr__`). But it doesn't feel quite right to me to return a multiline string for `__str__`. But following [the docs](https://docs.python.org/3.3/reference/datamodel.html#object.__str__), you could argue it's okay. I leave the decision up to you ;)

If you leave it as-is, I would just rename it to simply `print`, as the word `matrix` has become redundant now that it's a member function.

One last thing about this. I noticed that it still uses the Python 2 print syntax, so you probably haven't pulled in my latest changes (I only made them today). Might be worthwile to do it as soon as possible to avoid hairy conflicts...