Draft: Step 2 of redesign of quadrature rules

Summary

The classes QuadratureRule, QuadratureRules and QuadratureRuleFactory are cleaned up in style, using static map for storing rules and modern enum classes. The quadrature rules are put into a common programming style, e.g. no intermediate point/weight construction, but directly storing coefficients in the quadrature rule.

Details

Four major changes are introduced in this MR:

1. Quadrature rules (points and weights) are constructed directly and not via an intermediate c-array.
2. The quadrature rules are stored in a cache in form of a thread_local std::map. This allows to easily extend the key types (currently to topology-id, the quadrature order and quadrature type). In some initial tests there was no performance different measurable. It could be discussed whether it would be better to use a static std::map + locking, or whether another map, like an std::unordered_map would be the better choice.
3. The QuadratureType enum is now an enum class. This might be a breaking change in some use-code, if the quadrature type is stored somewhere. Needed changes in user code: replace Dune::QuadratureType::Enum by Dune::QuadratureType.
4. Instead of repeating all quarature points and weights for flating-point numbers and for string construction, a utility function/macro DUNE_NUMBER -> Dune::Impl::number is introduced to cover both cases at once. The macro takes a type and a floating-point literal and expands the latter into a double and a const char*. In the Dune::Impl::number template, it is then decided whether to construct from a string or returning the double value directly.

Motivation

This is the second step of a cleanup and redesign of the quadrature rules: 1.) put all rules into a similar structure. 2.) cleanup of the classes QuadratureRules and QuadratureFactory, 3.) Automatically generate coefficients for all rules with higher precision

The implementation intends to establish a common style that can be generated automatically in step 3 from a database.

Remarks

The simplexquadraturerule is transform from the old code using the attached python convert_rule.py and some postprocessing.