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These configurations are outdated for two reasons: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

CHOLMOD supports this, it just wasn't exposed in the interface.
It is needed for large problems.

Improve performance of MatrixIndexSet

See merge request !527

Using `std::uint_least32_t` instead of `std::size_t` halves the required memory
and thus also improves performance.
Notice that `std::uint32_t` is optional and `std::uint_fast32_t` is intended to be
fast in terms of computations and may have 64 bits. Here we're interested in reducing
memory and thus bandwith. Hence `std::uint_least32_t`, which is the smallest sufficient
type is most appropriate.

This replaces the `std::set` used to store the column indices for each
rows by a `std::vector` based implementation. The `std::vector` is
kept sorted when inserting such that we can avoid duplicates. This
can improve assembly time of matrices significantly (if `MatrixIndexSet` is used).

In the worst case (insertion in reverse) this may lead to O(n^2) complexity
when inserting n entries in a row compared to O(n log(n)) for `std::set`.
However, the sorted `std::vector` implementation still wins for relatively
large n. To avoid the worst case complexity when using very dense rows,
the implementation is switched to `std::set` if the size exceeds the threshold
value `maxVectorSize`. The default `maxVectorSize=2048` was selected based
on benchmark results.

Fix build guard for solverfactorytest_*_direct

See merge request !528

The CMAKE_GUARD for multiple guarding variables requires a
porper boolean statement.

Add FastAMG constructor with shared_ptr arguments

See merge request !524

Adjust sequential and both parallel cases in istl-solver-playground

See merge request !526