unconnected callouts in numpy lesson

[colinsauze]

There are several callouts in the numpy episode which are unconnected with the rest of the lesson. These include:

• comments
• format strings
• mystery functions
• not all functions have input

As this is already the longest episode these should be moved elsewhere or removed all together. Except for not all functions having input, we don't depend on knowing about any of these elsewhere in the lesson.