"Definition" in DefinedQuantityDict is not often a definition

[JacquesCarette]

PR #4298 reveals that quite often the information that is there is not a definition at all. Even when it is, the definition is not particularly useful. Certainly it is more often the case that the information is not useful to give out in code documentation.

What is often found is

• repetition of the term
• an expansion of the definition of one of the words in the term
• some local extra information (i.e. the term is generic but is used here in a more specialized manner)

Suggestion:

1. rename to extraInfo
2. create a data type of "extra information" that can contain a. extra (local) information b. a wordier version of the term c. nothing d. ?

Then we can choose which of this extra information should appear as code comments.