Cleanup CanIgnore interface

* Add constructor from bit vector
* Add methods to set/get the bit vector
* Improve doc

This is a first step in getting rid of the public member.
CanIgnore::ignoreNodes_ should be (add will hopefully become)
protected or private. With the new methods we can avoid
accessing the member directly.

dune/solvers/common/canignore.hh

@@ -13,17 +13,51 @@ public:
 
     using BitVector = BitVectorType;
 
-    /** \brief Default constructor */
+    /**
+     * \brief Default constructor
+     */
     CanIgnore()
         : ignoreNodes_(nullptr)
     {}
 
-    /** \brief Virtual destructor.  Does NOT delete the bitfield! */
+    /**
+     * \brief Constructor from bit vector
+     *
+     * This class stores a non-owning pointer to the bit vector.
+     */
+    CanIgnore(const BitVector& i)
+        : ignoreNodes_(&i)
+    {}
+
+    /**
+     * \brief Destructor
+     *
+     * Does NOT delete the bitfield!
+     */
     ~CanIgnore()
     {}
 
-    /** \brief A flag for each degree of freedom stating whether the
-        dof should be ignored by the solver */
+    /**
+     * \brief Set bit vector
+     *
+     * This class stores a non-owning pointer to the bit vector.
+     */
+    void setIgnore(const BitVector& i)
+    {
+        ignoreNodes_ = &i;
+    }
+
+    /**
+     * \brief Const access to bit vector
+     */
+    const BitVector& ignore() const
+    {
+        return *ignoreNodes_;
+    }
+
+    /**
+     * \brief A flag for each degree of freedom stating whether the dof should be ignored by the solver
+     */
     const BitVectorType* ignoreNodes_;
 
 };