[Bugfix] Prevent implicite conversion Matrix->MatrixAdapter.

MatrixAdapter uses a const reference to store a reference
to the matrix it is working on. Without this patch it was
possible to pass the matrix to constructor actually taking
a MatrixAdapter as its argument. This resulted in a const
reference to a temporary MatrixAdapter that died after the
constructor call.

This patch request explicitly calling the constructor and
thus fixes this issue.

dune/istl/operators.hh

@@ -131,7 +131,7 @@ namespace Dune {
     enum {category=SolverCategory::sequential};
 
     //! constructor: just store a reference to a matrix
-    MatrixAdapter (const M& A) : _A_(A) {}
+    explicit MatrixAdapter (const M& A) : _A_(A) {}
 
     //! apply operator to x:  \f$ y = A(x) \f$
     virtual void apply (const X& x, Y& y) const