Add DILU preconditioner

See merge request !547

fix an issue caused by generated files being added to source tree - move to a temporary location

Closes #111

See merge request !529

[cmake] Require CMake 3.16 or newer

See merge request !548

Initialize all members of SequentialInformation

See merge request !545

Resolve "writeSVGMatrix parameters are inconsistent with other IO writers"

Closes #104

See merge request !513

Resolve "Building istl-solver-playground causes compile time error"

Closes #112

See merge request !541

[foreach][cleanup] Only use return values in debug mode (silences warning)

See merge request !544

Add printSparseMatrix method for ScaledIdentityMatrix

See merge request !531

Fix error "delete  called on pointer returned from a mismatched allocation function"

See merge request !542

Move the find_package(SuiteSparse) from istl to common

See merge request !538

[ci] Drop no longer supported CI configurations

See merge request !540

These configurations are outdated for two reasons:

* Clang versions before 10 are no longer supported.
* Ubuntu 18.04 has reached its EOL in 2023-06.

UMFPACK: Use generic flatMatrixForEach routines for arbitrary blocked matrices

See merge request !530

CI failed before since the test matrix was singular.
This commit adds some regularization to pass tests on all runners.

Instead of calling the BCCSInitializer, we directly parse the matrix into
an UMFPACK compatible form. This allows us to use almost every blocked
matrix structure, such as MultiTypeBlockedMatrix. Moreover, the 'setMatrix'
method is extended by a second bitVector argument to handle exclusion of
indices. The old 'setSubMatrix' method is kept. The new interface is
inspired by the neighbored 'Cholmod' class.

The additional routines on the vector data are linear in time and have no
measurable influence on the run time of UMFPACK.

[cmake] Fix warning that mimimum required version has to come first

See merge request !535

Add an interface library for Dune::ISTL

See merge request !533

Cleanup the DUNECI_CMAKE_FLAGS for a failing job

See merge request !534

[doc] Enable doxygen for cholmod.hh

See merge request !532

So far doxygen did not process this file since
`HAVE_SUITESPARSE_CHOLMOD` was missing.
This also adds the missing editor hints.

Allow to use CHOLMOD with 'long int' integer type

See merge request !485