Rename FeaturesContainer to Classifier

[enikao]

Name is weird, because it describes the function of the element. Other M3 elements have names to describe their nature.

[enikao]

Postponed until #104 is decided (as we might not need FeaturesContainer if we don't have interfaces).

Consider future enhancement about Annotations (#13)

[enikao]

Postponed together with #104

[joswarmer]

Done on Juli 7 2023

[enikao]

On 2023-07-07, we decided to rename FeaturesContainer to Classifier.

Rationale: Name is weird, because it describes the function of the element. Other M3 elements have names to describe their nature.

Classifier is not ideal either, as the symmetric specialization of this would be Class rather than Concept. However, "classification" is a term in its own, and aptly fits this entity.
We discussed variants of Type (e.g. ObjectType) for name, symmetric to DataType. If we used Type instead of LanguageEntity (#147), there would be a nice symmetry. However, type is very easy to mix up with typesystem concerns, which is often very confusing.