make `FieldMatrix` and `FieldVector` trivially copyable types

Closes: #168 (closed), #169 (closed)

changed milestone to %DUNE 2.7.0

added 6 commits

• 87d9b800 - FieldMatrix should be trivially copyable and standard layout type
• 17dd2f17 - also test `FieldVector<T, 1>` specialization for 1-dim vectors
• 8cd10982 - FieldMatrix: mark default constructor as `constexpr`
• d0d5af50 - DiagonalMatrix: make default constructor `constexpr`
• eac2505d - Update CHANGELOG
• 85d57815 - FieldVector: do not use defaulted copy constructor with GCC 5.4

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resolved all discussions

mentioned in commit 433eac4e

merged