updated .gitignore;
updated .gitignore;
added notebook.txt for set01;
did exercise 11;
did exercise 12;
did exercise 13;
did exercise 14;
did exercise 15;
did exercise 16;
did exercise 17;
did exercise 18;
did exercise 19;
did exercise 20;
initial solution for exercise 21;
did exercise 21a;
did exercise 22;
did exercise 23;
did exercise 24;
did exercise 25;
initial solution for exercise 27;
did exercise 28;
did exercise 29;
finished exercise 21;