Draft: Add concept describing field types

Summary

This is an alternative to the type traits Dune::IsNumber where all types must be registered manually. It specifies what we understand under the term "field type". It is mathematically structured as a type with two operations + and *, which form additive or multiplicative groups.

In addition to the concept definition, the MR provides archetypes and placeholder functions for group operations identity() and inverse(). Those functions might be used in generic algorithms, e.g., to provide initial values.