Refactor smell warning to smell detector class mapping

[mvz]

Follow-up from #921:

End goal: Do not have both #smell_class and #smell_type instance methods for SmellWarning.

Steps:

• Allow retrieval of smell detector (class) from smell repository based on smell type string.
• Use this in TODO file generation.
• Prepare decoupling of smell detector from smell warning.
• Deprecate passing of smell detector to smell warning initializer.

Perhaps:

• Allow re-use of smell repository for all Examiner instances.

[troessner]

Sounds good to me!

[mvz]

The #smell_class method is gone now, so apparently this is done.