[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).

dune/fufem/backends/istlmatrixbackend.hh

@@ -86,7 +86,9 @@ namespace Impl {
     {
       assert(jjj.size()>0);
       auto [j, jj] = splitIndex(jjj);
-      return visitMatrixEntry(matrix, _0, j, [&](auto&& matrix_0j) -> decltype(auto) {
+      // We have to capture jj explicitly by value, because capturing structured bindings
+      // by reference is not allowed before C++20
+      return visitMatrixEntry(matrix, _0, j, [&, jj=jj](auto&& matrix_0j) -> decltype(auto) {
         return visitMatrixEntryRecursive(matrix_0j, iii, jj, f);
       });
     }
@@ -94,7 +96,9 @@ namespace Impl {
     {
       assert(iii.size()>0);
       auto [i, ii] = splitIndex(iii);
-      return visitMatrixEntry(matrix, i, _0, [&](auto&& matrix_i0) -> decltype(auto) {
+      // We have to capture ii explicitly by value, because capturing structured bindings
+      // by reference is not allowed before C++20
+      return visitMatrixEntry(matrix, i, _0, [&, ii=ii](auto&& matrix_i0) -> decltype(auto) {
         return visitMatrixEntryRecursive(matrix_i0, ii, jjj, f);
       });
     }
@@ -104,7 +108,9 @@ namespace Impl {
       assert(jjj.size()>0);
       auto [i, ii] = splitIndex(iii);
       auto [j, jj] = splitIndex(jjj);
-      return visitMatrixEntry(matrix, i, j, [&](auto&& matrix_ij) -> decltype(auto) {
+      // We have to capture ii and jj explicitly by value, because capturing structured bindings
+      // by reference is not allowed before C++20
+      return visitMatrixEntry(matrix, i, j, [&, ii=ii, jj=jj](auto&& matrix_ij) -> decltype(auto) {
         return visitMatrixEntryRecursive(matrix_ij, ii, jj, f);
       });
     }