[cleanup] Cleanup CanIgnore

* use nullptr
* use default naming scheme for include guard

dune/solvers/common/canignore.hh

 // -*- tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 // vi: set et ts=8 sw=4 sts=4:
-#ifndef CAN_IGNORE_HH
-#define CAN_IGNORE_HH
+#ifndef DUNE_SOLVERS_COMMON_CANIGNORE_HH
+#define DUNE_SOLVERS_COMMON_CANIGNORE_HH
 
 /** \brief Abstract base class for solvers and iterationsteps that are
  *      able to ignore degrees of freedom described by a bit field.
@@ -13,7 +13,7 @@ public:
 
     /** \brief Default constructor */
     CanIgnore()
-        : ignoreNodes_(0)
+        : ignoreNodes_(nullptr)
     {}
 
     /** \brief Virtual destructor.  Does NOT delete the bitfield! */
@@ -26,4 +26,4 @@ public:
 
 };
 
-#endif
+#endif // DUNE_SOLVERS_COMMON_CANIGNORE_HH