[cleanup]Simplify ISTLMatrixBackend entry access by multi-indices

This cleans up the entry access by a pair of row- and column-multi-indices as follows:

• Factor out a generic utility function visitMatrixEntry() for accessing the (i,j)-th entry of a matrix with dynamic indices. By combining hybridIndexAccess() and hybridRowIndexAccess() this also works with multi-type matrices.
• Factor out a generic utility function visitMatrixEntryRecursive() for recursively accessing matrix entries with dynamic row- and column-multi-indices. The action to apply to the resolved matrix entry is customizable.
• Use visitMatrixEntryRecursive() to return matrix entries.

This is a significant simplification of the old MultiIndexResolver-code and far more readable. It is also more generic, and forms the foundation of possible extensions to other methods that need multi-index-access, like, e.g., matrixEntryExists(i,j) or addToMatrixEntry(i,j).

This does some preliminary steps towards supporting more matrix types to get a solution for #25.

mentioned in issue #25

Currently the CI is expected to fail for testing against dune-istl master, because this requires the bugfix core/dune-istl!516 (merged) which needs to arrive in the CI image. For the tests against the releases this is not a problem, because the check uses a workaround in this case.

With updated CI images at least the gcc version compiles, while clang (correctly) still complains. I was surprised to learn, that a lambda is not allowed to capture structured bindings by reference before C++20. Capturing by value should be a valid workaround here.

added 1 commit

• b7682e10 - Capture structured bindings by value

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added 2 commits

• 63427735 - [test]Extend test of istlMatrixBackend
• 9630ef9c - Capture structured bindings by value

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mentioned in commit 79ec00b6

merged