[BCRSMatrix] Introduce new build stages notAllocated (aliased to

notbuilt) and building

Right now, some build modes reuse the same build stage value across
several actual stages. This patch splits the notbuilt stage into
notAllocated and building, avoiding that ambiguity.

dune/istl/bcrsmatrix.hh

@@ -415,15 +415,19 @@ namespace Dune {
     friend struct MatrixDimension<BCRSMatrix>;
   public:
     enum BuildStage {
-      /** @brief Matrix is not built at all. */
+      /** @brief Matrix is not built at all, no memory has been allocated, build mode and size can still be set. */
       notbuilt=0,
+      /** @brief Matrix is not built at all, no memory has been allocated, build mode and size can still be set. */
+      notAllocated=0,
+      /** @brief Matrix is currently being built, some memory has been allocated, build mode and size are fixed. */
+      building=1,
       /** @brief The row sizes of the matrix are known.
        *
        * Only used in random mode.
        */
-      rowSizesBuilt=1,
-      /** @brief The matrix structure is fully built.*/
-      built=2
+      rowSizesBuilt=2,
+      /** @brief The matrix structure is fully built. */
+      built=3,
     };
 
     //===== type definitions and constants