[cleanup] Stop using header arithmetic.hh

See merge request !18

The header ws once experimentally added to dune-fufem and
is superseded by `Dune::MatrixVector` itself. Hence we should
not use it here, but the analogues from `Dune::matrixVector`.

Disclaimer: This is not tested and I can't even find places
where I use it. Nevertheless the change is straight forward.

Constrain default transpose implementation

See merge request !15

The default implementation of `transpose()` returning
the original matrix should be constrained to plain numbers.

Specialize FieldTraits for custom matrix/vector types.

See merge request !17

This was made a hard requirement by a recent change in dune-istl.

[CI] Update CI

See merge request !16

The image `dune:git-debian-11-gcc-9-20` is no longer provided.
Use `dune:git-debian-11-gcc-10-20` instead.

Raise CMake requirements to 3.16

See merge request !14

The CMake in Debian 10 is not recent enough anymore.

A new build system warning tells me that the argument
is not needed anymore.

This is what the Dune core modules currently require.

[bugfix]Implement tuple protocol for custom type in resizetest.cc

See merge request !13

This fixes some fallout of the removal of `MultiTypeBlockMatrix::size()`:
By not providing a generic container interface you can no longer use generic
algorithms like indexed for-loops for matrices. One way to work around
this is to implement the `tuple` protocol to make `Hybrid::size()` work again.

For an alternative solution cf. core/dune-common!1209

GCC reported a compiler error. transformMatrix depends on transformMatrixPattern: therefore the latter should be implemented first.

Implement addToDiagonal for scalars (interpreted as 1x1 matrices)

See merge request agnumpde/dune-matrix-vector!12

The gcc-6 Docker image does not exist anymore.

This makes triangularsolvetest.cc pass.

This is required for dune-istl matrices than use scalars to end
the nesting recursion (and not FieldMatrix), like BCRSMatrix<double>.

 "Catching polymorphic type ‘class Dune::Exception’ by value"

Add missing header resize.hh to CMakeLists.txt

See merge request agnumpde/dune-matrix-vector!10

transpose.hh Fixed

See merge request agnumpde/dune-matrix-vector!8

Use modern CI script

See merge request agnumpde/dune-matrix-vector!6

This introduces the same CI script also used in the other agnumpde repositories.

See core/dune-common#77