Consistency for hidden code

[darribas]

Agreement is:

• Do not use code hiding if we refer to the code cell in the text/narrative
• Use it if the code is not referred to in the text, but add a note on the caption to notify the reader

Text to use in the caption:

Code generated for this figure is available on the web version of the book.

This needs to be implemented across all chapters.

[darribas]

Also make sure there is no removed code, that non-visible code is done through hidden.

[darribas]

Addressed in #216, closing...