Make Geometry export the return type of the 'volume' method

Previously this was 'ctype', but grids may care about proper
dimensions.  In that case, a volume is not the same as a
coordinate, and a separate type for volumes is necessary.

The interface class does not hard-wire any type but rather exports
the type used by the Geometry implementation class.  That type
is determined by introspection, there is no need to adapt
downstream grid implementations.

CHANGELOG.md

@@ -64,6 +64,12 @@
 - The Capability `hasBackupRestoreFacilities<GeometryGrid<HG, CoordFunction>>` 
   now returns `false` in case the `CoordFunction` is not default-constructible.
 
+- The `Geometry` interface now provides the type `Volume` for the return value of the
+  method of the same name.  Note that this may be different from `ctype` if you care
+  about dimensions.  In that case `ctype` is a length, and not appropriate for
+  a quantity that is a volume.
+
+
 # Release 2.6
 
 - The deprecated `EntityPointer` has been removed completely and `EntityIterator`

dune/grid/common/geometry.hh

@@ -100,6 +100,9 @@ namespace Dune
     //! type of the global coordinates
     typedef FieldVector< ctype, cdim > GlobalCoordinate;
 
+    //! Number type used for the geometry volume
+    typedef decltype(std::declval<Implementation>().volume()) Volume;
+
     /**
      * \brief type of jacobian inverse transposed
      *
@@ -202,7 +205,7 @@ namespace Dune
     }
 
     /** \brief return volume of geometry */
-    ctype volume () const
+    Volume volume () const
     {
       return impl().volume();
     }
@@ -307,6 +310,9 @@ namespace Dune
     typedef FieldVector< ctype, mydim > LocalCoordinate;
     typedef FieldVector< ctype, cdim > GlobalCoordinate;
 
+    //! Number type used for the geometry volume
+    typedef ctype Volume;
+
     //! type of jacobian inverse transposed
     typedef FieldMatrix< ctype, cdim, mydim > JacobianInverseTransposed;
 
@@ -314,7 +320,7 @@ namespace Dune
     typedef FieldMatrix< ctype, mydim, cdim > JacobianTransposed;
 
     //! return volume of the geometry
-    ctype volume () const
+    Volume volume () const
     {
       GeometryType type = asImp().type();
 
@@ -365,6 +371,7 @@ namespace Dune
 
     typedef FieldVector< ctype, mydim > LocalCoordinate;
     typedef FieldVector< ctype, cdim > GlobalCoordinate;
+    typedef ctype Volume;
 
     //! type of jacobian inverse transposed
     typedef FieldMatrix< ctype, cdim, mydim > JacobianInverseTransposed;
@@ -385,9 +392,9 @@ namespace Dune
     }
 
     //! return volume of the geometry
-    ctype volume () const
+    Volume volume () const
     {
-      return 1.0;
+      return Volume(1.0);
     }
 
     //! return center of the geometry