Migration with default values

The new method will add child elements into the migrated node based on the input port count, such as:
```
<Dynamo.Nodes.DSFunction type="Dynamo.Nodes.DSFunction" nickname="Point.ByCoordinates" ... >
  <PortInfo index="0" default="True" />
  <PortInfo index="1" default="True" />
  <PortInfo index="2" default="True" />
</Dynamo.Nodes.DSFunction>
```