Rework Feature API

[TobyBoyne]

Currently, there are a few issues with the feature API:

• Instance methods: the methods such as is_cat() should be class attributes, not instance methods, as they do not require processing and do not vary between instances
• Validation: currently, validation is performed in the ProblemConfig.add_feature method. The responsibility to validate should belong to the feature
• Naming: each class should have Feature at the end to make it clear that this is a custom object for representing features.

Each of these changes would break the existing API, so I am deferring these changes until the next major release. A good solution would use attrs to create dataclasses.

We should also consider whether using an existing API like bofire would be feasible.