add mid math

modules/math/mid.js

 import Position from '@dc-modules/position/Position'
 
 export default function mid(start, end) {
-  let startPosition = Parse.parsePosition(start)
-  let endPosition = Parse.parsePosition(end)
-  startPosition = Transform.transformWGS84ToCartographic(startPosition)
-  endPosition = Transform.transformWGS84ToCartographic(endPosition)
-
+  let startPosition = start
+  let endPosition = end
+  if (!(start instanceof Cesium.Cartesian3)) {
+    startPosition = Parse.parsePosition(start)
+    startPosition = Transform.transformWGS84ToCartesian(startPosition)
+  }
+  if (!(end instanceof Cesium.Cartesian3)) {
+    endPosition = Parse.parsePosition(end)
+    endPosition = Transform.transformWGS84ToCartesian(endPosition)
+  }
   let mc = new Cesium.EllipsoidGeodesic(
     startPosition,
     endPosition