On the way to a 130/130 pub.dev score...

Impacted files:
* `ContainsAdditives.dart`: added a comment on class; refactored as non-nullable
* `OutputFormat.dart`: added a comment on class and enum; refactored
* `Page.dart`: added a comment on class; refactored as non-nullable
* `PageSize.dart`: added a comment on class; refactored as non-nullable
* `Parameter.dart`: added a comment on class; created method `addToMap`
* `ProductSearchQueryConfiguration.dart`: added a comment on class; refactored some fields as non-nullable
* `SearchSimple.dart`: not done much as the purpose of the class is unclear
* `SearchTerms.dart`: added a comment on class; refactored as non-nullable
* `SortBy.dart`: added a comment on class and enum; refactored
* `TagFilter.dart`: added a comment on class and enum; overridden new method `addToMap`; refactored

I've just rolled back changes I made that actually added complexity without added value.

Impacted files:
* `Parameter.dart`
* `ProductSearchQueryConfiguration.dart`
* `TagFilter.dart`