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/FlowWorkOptions.cs

@@ -20,12 +20,12 @@ namespace Serein.NodeFlow
         /// <summary>
         /// 流程起始节点
         /// </summary>
-        public Func<NodeModelBase> GetStartNode { get; set; }
+        public Func<IFlowNode> GetStartNode { get; set; }
 
         /// <summary>
         /// 获取当前画布流程的所有节点
         /// </summary>
-        public Func<List<NodeModelBase>> GetNodes { get; set; }
+        public Func<List<IFlowNode>> GetNodes { get; set; }
     }
 
     /// <summary>