model: Rename instance attributes to not contain trailing underscores

Trailing underscores are nice for function parameters (including initializer parameters) to avoid name conflicts with global denominators (like module names and builtins). However, instance attributes form an own namespace and don't conflict with global denominators. Thus, trailing underscores are not required for them and I would prefer removing them to have less confusing attribute names.

However this should not be changed, before we merged the features/json_adapter branch.