remove keyword explicit for the bool cast of shared_ptr

[[Imported from SVN: r7041]]

dune/common/shared_ptr.hh

@@ -113,7 +113,7 @@ namespace Dune
     }
 
     /** \brief Checks if shared_ptr manages an object, i.e. whether get() != 0. */
-    explicit operator bool() const {
+    operator bool() const {
       return rep_ != 0 && rep_->rep_ != 0;
     }