Add the checktree routines for Morphological Tree Sanity Check.

    *  mln/morpho/datastruct/checktree.hpp: New.

mln/morpho/datastruct/checktree.hpp

+#ifndef MLN_MORPHO_DATASTRUCT_CHECKTREE_HPP
+# define MLN_MORPHO_DATASTRUCT_CHECKTREE_HPP
+
+# include <mln/morpho/datastruct/component_tree.hpp>
+
+namespace mln
+{
+  namespace morpho
+  {
+    namespace internal
+    {
+
+      template <class Tree>
+      void
+      checktree(const Tree& tree)
+      {
+        typename Tree::node_type prec = tree.nend();
+
+        mln_foreach(auto node, tree.nodes())
+          {
+            mln_assertion(node.get_parent_id() == prec.id() or
+                          prec.get_next_sibling_id() == node.id());
+
+            mln_assertion(node.next_node().get_parent_id() == node.id() or
+                          node.get_next_node_id() == node.get_next_sibling_id());
+
+            mln_assertion(node.next_sibling().get_parent_id() == node.get_parent_id() or
+                          node.get_next_sibling_id() == node.parent().get_next_sibling_id());
+
+            prec = node;
+          }
+
+      }
+    }
+  }
+}
+
+#endif // ! MLN_MORPHO_DATASTRUCT_CHECKTREE_HPP