改写NodeModelBase类，使其继承Serein.Library.Api下的IFlowNode接口，而实现类迁移到NodeModel项目，方便后续节点运行逻辑修改时不用重新编译类库。

NodeFlow/Env/FlowEnvironmentDecorator.cs

@@ -560,7 +560,7 @@ namespace Serein.NodeFlow.Env
             currentFlowEnvironment.SetUIContextOperation(uiContextOperation);
         }
 
-        public bool TryGetNodeModel(string nodeGuid, out NodeModelBase nodeModel)
+        public bool TryGetNodeModel(string nodeGuid, out IFlowNode nodeModel)
         {
             return currentFlowEnvironment.TryGetNodeModel(nodeGuid, out nodeModel);
         }