Add notebook for item 31: Use Descriptors for Reusable @property Methods

[jekwatt]

Notebook for item 31:

• item_31_use_descriptors

[jekwatt]

See the below for related subjects:

• item 26 Add notebook for item 26: Use Multiple Inheritance Only for Mix-in Utility Classes #88
• item 29 Add notebook for item 29: Use Plain Attributes Instead of Get and Set Methods #89
• item 32 Add notebook for item 32: Use __getattr__, __getattribute__, and __setattr__ for Lazy Attributes #92