wrote notes for chapter 27
updated global notes
refactored solutions to exercises
refactored chapter 6
refactored chapter 7 exercises
refactored chapter 9 exercises
refactored exercises for chapter 12
refactored chapter 13 exercises
solved exercise 21.4
added solutions to exercise 21.4 to wrong folder
fixed spelling
solved exercise 21.5
added tests for exercise 19.1
added a todo list