Feature/organize traits

I'd like to keep things organized before they get out of hand.

This puts the traits structs to a dedicated namespace Traits, but leaves the traits utilities in the namespace Dune::MatrixVector. It makes use of the traits utilities in several places (axpy, axy, transformmatrix), which IMHO improves readability a lot. Adds deprecation warnings for the moved files.

Additionally this extend traits that the core modules are lacking, e.g. FieldTraits for ScaledIdentityMatrix.

added 2 commits

• cde845c0 - Add decay to compile-time classifiers.
• f274a928 - Add workaround for compilers not deducing constexpr as expected.

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added 3 commits

• 1d64d27b - Move traits classes and utilities to separate directory.
• fc3da5dd - Adapt to moved traits and use traits utilities.
• cd99eee3 - Add deprecation warnings for old matrixtraits and scalartraits includes.

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changed the description

added 2 commits

• 952c54cd - Adapt to moved traits and use traits utilities.
• 64434c48 - Add deprecation warnings for old matrixtraits and scalartraits includes.

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added 2 commits

• 4d859e3d - Provide FieldTraits that are missing in the core modules.
• 1d51d3e7 - Remove dummy values from matrix traits in favor of compile-time errors.

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changed the description

assigned to @maxka

merged

mentioned in commit 93000bba