Add virtual destructors to Dumux::Vtk::Field and Dumux::NewtonSolver

This fixes a delete-non-virtual-dtor warning from Clang.

dumux/io/vtkfunction.hh

@@ -260,6 +260,8 @@ public:
             DUNE_THROW(Dune::NotImplemented, "Only element or vertex quantities allowed.");
     }
 
+    virtual ~Field() {}
+
     //! return the name of this field
     virtual std::string name () const { return field_->name(); }
 

dumux/nonlinear/newtonsolver.hh

@@ -110,6 +110,8 @@ public:
             partialReassembler_ = std::make_unique<Reassembler>(*assembler_);
     }
 
+    virtual ~NewtonSolver() {}
+
     //! the communicator for parallel runs
     const Communication& comm() const
     { return comm_; }