Introduce method 'fixedSize', which does the same as 'fixedsize', but has camel-case naming

The CommDataHandleIF interface class has a member 'fixedsize'.  This is one
of the few in dune-grid that does not comply with the official naming rules:
the name should be 'fixedSize'.  This patch introduces a new method 'fixedSize',
to allow a gradual transition from the old spelling to the new one.
The make the transition as painless as possible, but methods implement the
following behavior (remember that CommDataHandleIF is the base class in a
CRTP pattern):

- If the derived/implementation class has a method 'fixedSize', then that
  method is called.
- Otherwise, the method 'fixedsize' is called.

This approach allows both implementors of grids and implementors of data
handles to switch their code from 'fixedsize' to 'fixedSize' at their
convenience.

Finally, I have come across a usecase for CRTP. :-)

dune/grid/common/datahandleif.hh

@@ -95,11 +95,38 @@ namespace Dune
        returns true if size of data per entity of given dim and codim is a constant
        @param dim valid dimension (i.e. the grids dimension)
        @param codim valid codimension of the entity set for which data should be communicated
+
+       This method calls 'fixedSize' (with a capital S) of the derived class,
+       if it exists in the derived class.  Otherwise, it calls 'fixedsize'.
      */
     bool fixedsize (int dim, int codim) const
     {
-      CHECK_INTERFACE_IMPLEMENTATION((asImp().fixedsize(dim,codim)));
-      return asImp().fixedsize(dim,codim);
+      auto basePtr = &CommDataHandleIF<DataHandleImp,DataTypeImp>::fixedSize;
+      auto derPtr = &DataHandleImp::fixedSize;
+      bool hasOverwrittenFixedSize = basePtr != derPtr;
+      if (hasOverwrittenFixedSize)
+        return asImp().fixedSize(dim,codim);
+      else
+        return asImp().fixedsize(dim,codim);
+    }
+
+    /** @brief
+       returns true if size of data per entity of given dim and codim is a constant
+       @param dim valid dimension (i.e. the grids dimension)
+       @param codim valid codimension of the entity set for which data should be communicated
+
+       This method calls 'fixedSize' (with a capital S) of the derived class,
+       if it exists in the derived class.  Otherwise, it calls 'fixedsize'.
+     */
+    bool fixedSize (int dim, int codim) const
+    {
+      auto basePtr = &CommDataHandleIF<DataHandleImp,DataTypeImp>::fixedSize;
+      auto derPtr = &DataHandleImp::fixedSize;
+      bool hasOverwrittenFixedSize = basePtr != derPtr;
+      if (hasOverwrittenFixedSize)
+        return asImp().fixedSize(dim,codim);
+      else
+        return asImp().fixedsize(dim,codim);
     }
 
     /** @brief how many objects of type DataType have to be sent for a given entity