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/__init__.py

@@ -0,0 +1,30 @@
+"""This package provides linear algebra functionalities.
+
+The package is split into three modules:
+- matrix: defines the Matrix class
+- vector: defines the Vector class
+- utils: defines the norm() function that is shared by Matrix and Vector
+         and package-wide constants
+
+The classes implement arithmetic operations involving vectors and matrices.
+
+See the docstrings in the modules and classes for further info.
+"""
+
+# Import the classes here so that they are available
+# from the package's top level. That means that a user
+# who imports this package with `import sample_package`
+# may then refer to, for example, the Matrix class with
+# simply `sample_package.Matrix` instead of the longer
+# `sample_package.matrix.Matrix`.
+from sample_package.matrix import Matrix
+from sample_package.vector import Vector
+
+
+# Define meta information for the package.
+# There are other (and more modern) ways of
+# doing this, but specifying the following
+# dunder variables here is the traditional way.
+__name__ = "linear_algebra_tools"
+__version__ = "0.1.0"  # see https://semver.org/ for how the format works
+__author__ = "Alexander Hess"