[cleanup] Cleanup BitVector

* There are no virtual methods, hence we don't need a virtual destructor
* Export BitVector type

dune/solvers/common/canignore.hh

@@ -11,13 +11,15 @@ class CanIgnore
 {
 public:
 
+    using BitVector = BitVectorType;
+
     /** \brief Default constructor */
     CanIgnore()
         : ignoreNodes_(nullptr)
     {}
 
     /** \brief Virtual destructor.  Does NOT delete the bitfield! */
-    virtual ~CanIgnore()
+    ~CanIgnore()
     {}
 
     /** \brief A flag for each degree of freedom stating whether the