add metadata setter

Author: mpsonntag

+nix/MetadataMixIn.m

@@ -23,7 +23,17 @@
                 strcat(obj.alias, '::openMetadataSection'), ...
                 obj.nix_handle, obj.metadataCache, @nix.Section);
         end;
-    end
+
+        function set_metadata(obj, val)
+            if (isempty(val))
+                nix_mx(strcat(obj.alias, '::set_none_metadata'), obj.nix_handle, val);
+            else
+                obj.metadataCache = nix.Utils.add_entity(obj, val, 'nix.Section', ...
+                    strcat(obj.alias, '::set_metadata'), obj.metadataCache);
+            end;
+            obj.metadataCache.lastUpdate = 0;
+        end;
+    end;
 
 end