Ensure ##NEW convention is consistent throughout

[Bergam0t]

New code has been identified with ##NEW throughout in code snippets (in line with how it's done in lectures and code examples this is based off).

Need to check for spacing and consistency of double # throughout to ensure it's easy to find the code additions, rather than a mix of #NEW, # NEW, ##NEW and ## NEW that I think has crept in.

[amyheather]

Uses old format of indicators:

• 15.2 - a few examples of "## NEW" and "##MODIFIED"
• 15.3 - a few examples of "# NEW", "## NEW", and "##MODIFIED"
• 17.2.1.3.2 - "##EDITED"
• 23.2.3 - a few examples of "## NEW"

Missing a new indicator:

• 15.2.5 - does this need a "##NEW"?
• 17.2.1.2.2 - does this need a "##NEW"? (appreciate it says that its a new method, but if someone if looking at whole code and doing Ctrl+F to find new sections would probably be handy)
• 23.2 - missing "##NEW" for some sections (e.g. g, Patient)
• 23.2.5.2