Extensions: decorators

[barneydobson]

Description

My demonstration of decorators in the extensions file.

I have implemented it on one of the example tutorials.

If we're happy with it I think we will have two further issues:

• Decorators behaviour, where further functions are added in extensions (e.g., extend.surface, etc.)
• Decorators notebook, where we update existing tutorials with this new usage (they need updating anyway)

I think we will also want a notebook for extensions in general, but for me it makes sense to do this after we support subclasses via it.

[dalonsoa]

Supersedded by #101