[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).