Following discussion on how to let Time interact with units without relying overly on ducktyping, @embray suggested (#3648 (comment)) a Quantity abstract base class, with which objects could register to let other code know they can be treated as quantities with a unit. For Unit itself, this would also be useful, in particular for things like functional units which share many but not all of the properties of normal units (see
#1894 (comment)). This issue is to remind us of these thoughts.

p.s. Such a base class might also go quite a long way towards ensuring interoperability with other unit implementations (such as that by amuse).