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/utils.py

@@ -0,0 +1,14 @@
+"""This module provides utility functions."""
+
+
+def norm(vector_or_matrix):
+    """Calculate the Frobenius or Euclidean norm of a matrix or vector.
+
+    Args:
+        vector_or_matrix (Vector/Matrix): the entries whose squares
+            are to be summed up
+
+    Returns:
+        norm (float)
+    """
+    return math.sqrt(sum(x ** 2 for x in vector_or_matrix))