Introduce Chars

This improves the implementation by introducing a class to manage the characters

setup.cfg

 [metadata]
 name = ilp_keyboard_layout_optimization
-version = 0.0.2a7
+version = 0.0.2a9
 description =
     The QAP variant of keyboard layout optimization, i.e. character to key assignments
 long_description = file: README.md
@@ -41,3 +41,8 @@ python_requires = >=3.10
 
 [options.packages.find]
 where = src
+
+[options.extras_require]
+test =
+    hypothesis
+    pytest