Code base refactoring

[M123-dev]

Tracked in

https://github.com/openfoodfacts/openfoodfacts-dart/projects/2

Why

The code of the sdk has grown a lot lately and will probably continue to do so, albeit at a slower pace. To keep the code maintainable a few things will probably have to be changed. The things mentioned here are not decided, only ideas and are open for discussion.

What

• Update tests so that they do not expect specific values from existing products on openfoodfacts.net, unless they write them first #232
• Confusion with Product field priorities and read/write statuses #291
• Rewriting documentation #342
• Further splitting up of openfoodfacts.dart #339

[monsieurtanuki]

To me the biggest issue is with #291. I can add fields to Product but I don't know what they're here for, the typical values, what's their unit (if applicable), if they work in READ, in WRITE (#331 (comment)), if they are impacted by the states tags (cf. #326). Comments on each field would help, at least in a first approach.