Fix typo.
Delete mention of skimage library.
Change calculation of cos(pi/2) from a demo to an exercise.
Delete explanation of picture.show() method.
Delete spurious cell output.
Correct variable name.
Delete mention of skimage library.
Change several demos to exercises.

* Playing with the ipythonblocks.show_color() function
* Creating a 20x8 grid with a raspberry left column
* Creating a 20x3 grid with columns 5, 10 & 15 colored ochre
* Inverting the nested loops to color th upper half of the square coral
Delete discussion of try/except blocks and exception handling.