File usage decorators

Adds decorators to allow for local file use.

Usage:

• The @use_files decorator can be used on a function, temporarily changing the working directory to the folder of the file in which the function is called. After the execution of the function the workdir is changed back to the original workdir.
• The @use_files_class decorator can be used on a class. The workdir is changed to the folder of the file in which the class is defined before instantiating the class.