Add helper to use stack variables with shared_ptr

If an interface requires a shared_ptr, but we have an object that is
known to outlive the client anyway, we can use a custom deleter to
suppress the delete part and pass this object around anyway.

opm/core/io/OutputWriter.hpp

@@ -31,6 +31,20 @@ namespace parameter { class ParameterGroup; }
 class SimulatorTimer;
 class WellState;
 
+/*!
+ * Custom deleter that does nothing. This is usable when an interface
+ * needs a shared_ptr, but you want to pass an object that has local
+ * storage (and you know that the shared_ptr client doesn't need it
+ * outside of the scope).
+ *
+ * \example
+ * \code{.cpp}
+ *  Foo obj;
+ *  auto ptr = std::shared_ptr <Foo> (&obj, no_delete);
+ * \endcode
+ */
+inline void no_delete (void const *) { }
+
 /*!
  * Interface for writing non-compositional (blackoil, two-phase) simulation
  * state to files.
@@ -85,6 +99,8 @@ public:
      *               that is returned.
      *
      * @return       Pointer to a multiplexer to all applicable output formats.
+     *
+     * @see Opm::no_delete
      */
     static std::unique_ptr <OutputWriter>
     create (const parameter::ParameterGroup& params,