Add Number concept in terms of IsNumber

Summary

We use the type traits Dune::IsNumber to define leaf nodes element-types in (hierarchically nested) containers, e.g., to have break conditions for recursive algorithms and a restriction which scalar types can be used in constant initialization and scalar multiplication of these containers. This type trait is moved into a concept. This is a preparation for a later more specific characterization of the semantics of such (scalar) number types.

Applications

While one could also use the traits class IsNumber, a concept gives a little bit syntactic sugar. We could write

template <class T, int n>
auto operator(const auto Concept:Number& factor, const FieldVector<T,n>& vector);
// or
template <Concept:Number N, class T, int n>
auto operator(const N& factor, const FieldVector<T,n>& vector);

Later, we could maybe be more precise about the number type, e.g., using Concept::Field (!1494)

added concepts suggestion labels

In general, I'd prefer having the change log entry together with the actual change in code. It helps me to figure out changes when I adopt other modules and do not understand (anymore) the change log entry.

mentioned in merge request !1401

When we introduces IsNumber we deliberately did not call it IsScalar because the standard already has a concept of 'scalars' (std::is_scalar) which is neither a subset nor a superset of our IsNumber: It e.g. includes pointer-to-members but must not be specialized for custom types representing numbers.

The pros and cons of IsScalar have already been discussed and let to IsNumber. Hence the concept introduced here should be called Number.

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changed title from Add Scalar concept in terms of IsNumber to Add Number concept in terms of IsNumber

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mentioned in commit 6573ed38