Add method numLevels() which returns, well, the number of levels

Note: it does _not_ return the content of the numLevels_ variable,
but rather the number of transfer operators plus one.

[[Imported from SVN: r7397]]

dune/solvers/iterationsteps/multigridstep.hh

@@ -187,6 +187,12 @@
 //        virtual VectorType getSol();
 
 //        virtual const MatrixType* getMatrix();
+        
+        /** \brief Return total number of levels of the multigrid hierarchy */
+        virtual size_t numLevels() const
+        {
+            return mgTransfer_.size() + 1;
+        }
 
         virtual int level() const {return level_;}