优化更新代码，添加界面功能并整合

ShrlAlgo.Toolkit.Revit/Assist/LevelAssist.cs

@@ -0,0 +1,12 @@
+using Autodesk.Revit.DB;
+
+namespace ShrlAlgo.Toolkit.Revit.Assist
+{
+    public static class LevelAssist
+    {
+        public static SketchPlane GetLevelSketch(this Level level)
+        {
+            return SketchPlane.Create(level.Document, level.GetPlaneReference());
+        }
+    }
+}