[release][bcrsmatrix][bugfix] make data of BCRSMatrix protected to allow derivation and

modification of member data. This is needed in OPM.

dune/istl/bcrsmatrix.hh

@@ -1891,7 +1891,7 @@ namespace Dune {
     }
 
 
-  private:
+  protected:
     // state information
     BuildMode build_mode;     // row wise or whole matrix
     BuildStage ready;               // indicate the stage the matrix building is in