Add the sample_package folder to the project

The sample_package folder serves as an example in notebook 02 as to what
a Python package looks like. Its contents actually belong to the chapter
on object-orientation.

sample_package/__init__.py

@@ -0,0 +1,5 @@
+"""This package provides Vectors and Matrices."""
+
+from .matrix import Matrix
+from .utils import norm
+from .vector import Vector