Aesthetics fixes, variable names changes, markdown update

[pmiquelm]

A couple broken tutorials were fixed. Several variable names were changed for improved clarity as a well as compatibility with the older version of the interpreter. The syntax ~~~ was replaced with the more universal Markdown syntax ```. Several bug fixes and improvements were made to the convertonotebook script. this includes capitalizing the first letter in every Markdown cell, and ensuring the link in the header is reproduced appropriately. Lastly, many aesthetic improvements were made to the tutorials, including ensuring tabs are three spaces, and styling important comments with Markdown.

[peremato]

Thanks very much. Merged into master.