Draft: Add a view to raw vector data with `DenseVector` interface

Summary

This MR introduces a span DenseVectorSpan over a consecutive sequence of values with a given number. This span has an interface like a DenseVector and can be used in all the places where a DenseVector is expected, e.g. in DenseMatrix::solve. This allows to wrap non-Dune types like std::vector in the Dune interface. It also would allow to replace the ScalarVectorView since a single scalar value is just a sequence of length 1.

Requirements

• This MR is based on !1334 (merged)

Note

A similar span can be easily introduced for matrices. A generalization is already implemented for "tensors" in https://gitlab.dune-project.org/simon.praetorius/dune-tensor/-/blob/main/dune/tensor/tensorspan.hh?ref_type=heads

dune/common/test/CMakeLists.txt

@@ -118,6 +118,9 @@ endforeach()
 dune_add_test(SOURCES densevectorassignmenttest.cc
               LABELS quick)
 
+dune_add_test(SOURCES densevectorspantest.cc
+              LABELS quick)
+
 dune_add_test(SOURCES diagonalmatrixtest.cc
               LABELS quick)