Simplify redundant information in configuration

This PR implements #106 (closed) to simplify some of the redundant information stored in different parts of the configuration. In particular, two different changes/simplifications were made:

• [job.data_discovery] -> [features]
• Inherit column information for classifier sections from [features]

This should avoid some of the error-prone configuration that is repeated across multiple sections and instead stored in a single place.

Closes #106 (closed).