This PR restructures some elements of feos-core that make the code easier and a bit less opinionated.

• Removal of the methods that return trait objects from Residual and IdealGas: the interface for the Helmholtz energy calculation no longer distinguishes between different contributions.
• Consequently, we get rid of the traits and constructs to create object-safe traits.
• Since the interface for the Helmholtz energy is less rigid we can think about efficiently calculating properties that are used across different contributions or even caching.