Update debugging lesson

[jarrodmillman]

I did some basic updates in #740, but this section needs a larger overhaul including:

• replace nosetest with pytest
• the wiener_filtering.py debugging example
  (To get it to work I had to use np.int64 casting, which makes the example contrived.
  A better example should be found and the text updated.)
• the Debugging segmentation faults using gdb needs to be redone
  (The segfault.py example needs to be replaced.)