Make `LeftAndMain` abstract and other small refactoring in this class

There's no good reason to instantiate LeftAndMain. It should be abstract.

Acceptance criteria

LeftAndMain is made abstract
Anything that requires instantiating LeftAndMain is refactored to use a concrete class instead.
A new abstract getModelClass() method is created
The tree_class configuration property is removed - anything that used this configuration is updated to call getModelClass() instead.
Methods which used that config are updated according to the "tree_class config" section in this comment unless doing so causes unforeseen problems
Recommendations in the "Additional recommendations" section in this comment are applied unless doing so causes unforeseen problems