Merge branch 'feature/fixes-from-exadune' into 'master'

[diagonalmatrix] Do return from function that returns non-void.

This is a bugfix from exadune that hadn't made it upstream yet.

See merge request !68

dune/common/diagonalmatrix.hh

@@ -1012,6 +1012,7 @@ namespace Dune {
     {
       containerWrapper_ = other.containerWrapper_;
       position_ = other.position_;
+      return *this;
     }
 
     // This operator is needed since we can not get the address of the