Add sample_package with linear algebra tools

- add sample_package:
  + __init__.py => high-level description and imports
  + matrix.py => define a Matrix class
  + vector.py => define a Vector class
  + utils.py => package-wide utilities
- streamline the code snippets in the chapter 11 notebooks
  to align with the sample_package

11_classes/sample_package/utils.py

@@ -0,0 +1,35 @@
+"""This module provides utilities for the whole package.
+
+The defined constants are used as defaults in the Vector and Matrix classes.
+
+The norm() function is shared by Vector.__abs__() and Matrix.__abs__().
+"""
+
+import math
+
+
+# Define constants (i.e., normal variables that are, by convention, named in UPPERCASE)
+# that are used as the defaults for class attributes within Vector and Matrix.
+DEFAULT_ENTRIES_STORAGE = tuple
+DEFAULT_ENTRY_TYPE = float
+ZERO_THRESHOLD = 1e-12
+
+
+def norm(vec_or_mat):
+    """Calculate the Frobenius or Euclidean norm of a matrix or vector.
+
+    Find more infos here: https://en.wikipedia.org/wiki/Matrix_norm#Frobenius_norm
+
+    Args:
+        vec_or_mat (Vector / Matrix): object whose entries are squared and summed up
+
+    Returns:
+        norm (float)
+
+    Example Usage:
+        As Vector and Matrix objects are by design non-empty sequences,
+        norm() may be called, for example, with `[3, 4]` as the argument:
+        >>> norm([3, 4])
+        5.0
+    """
+    return math.sqrt(sum(x ** 2 for x in vec_or_mat))