[!268] Implement MatrixDimension for dense Matrices

Merge branch 'implement-printmatrix-matrix-double' into 'master'

See merge request [!268]

  [!268]: Nonecore/dune-istl/merge_requests/268
6 jobs for master in 11 minutes and 24 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #75399
duneci
debian:10 clang-7-libcpp-17

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passed #75398
duneci
debian:10 gcc-8-noassert-17

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passed #75401
duneci
debian:9 clang-3.8-14

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passed #75400
duneci
debian:9 gcc-6-14

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passed #75402
duneci
ubuntu:16.04 gcc-5-14

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passed #75403
duneci
ubuntu:18.04 clang-6-17

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