Added plot of trailing edge without circulation

A source-panel-only solution with non-zero angle of attack causes a
strange streamline pattern at the trailing edge ...
lesson 10, with 40 panels, no changes

it just wants to commit because of the changed prompt numbers and png
code.
Adding more content, especially Kutta condition

Rearranging the text and adding more explanations, plus the PNG for a
source-only solution with non-zero angle of attack