Rip out NumPy and replace with small vector class

This change removes NumPy and makes blurring with SciPy optional.

Here's my rationale:
- NumPy and SciPy are pretty huge
- I want to run Shaape on small machines such as the RaspberryPi from source
- NumPy wasn't even used that heavily

All the tests succeed, except for one where a generated image was committed. It might make also sense to adapt the `Node` class.