Add notebook for item 30: Consider @property Instead of Refactoring Attributes

[jekwatt]

Notebook for item 30:

• item_30_use_property

[jekwatt]

See the below for related subjects:

• item 22 Add notebook for item 22: Prefer Helper Classes Over Bookkeeping with Dictionaries and Tuples #56
• item 29 Add notebook for item 29: Use Plain Attributes Instead of Get and Set Methods #89