some conversion operators and sqrt in namespace std for the mpf_class wrapper

[[Imported from SVN: r5945]]

dune/common/gmpfield.hh

@@ -35,6 +35,23 @@ namespace Dune
        return *this;
        }
      */
+
+    // type conversion operators
+    operator double () const
+    {
+      return this->get_d();
+    }
+
+    operator float () const
+    {
+      return this->get_d();
+    }
+
+    operator mpf_class () const
+    {
+      return static_cast<const mpf_class&>(*this);
+    }
+
   };
 
 
@@ -90,6 +107,16 @@ namespace Dune
 
 }
 
+namespace std
+{
+  template< unsigned int precision >
+  inline Dune::GMPField< precision >
+  sqrt ( const Dune::GMPField< precision > &a )
+  {
+    return Dune::GMPField< precision >(sqrt(static_cast<const mpf_class&>(a)));
+  }
+}
+
 #endif // HAVE_GMP
 
 #endif // #ifndef DUNE_MULTIPRECISION_HH