GraphEditor / Viewer : uv set `index` value always 0 for <texcoord> nodes

Not sure why but if you have a `` node in your document and load it in, the `index` value is always set to 0.
Tried this with both the GraphEditor and Viewer

Just loading in the file and dumping the string for it seems fine so it's not a file I/O issue.

Not sure why this would be, but seems very strange. Tried other nodes with integer inputs and they seem fine.

This is a sample file with + bunch of other nodes with integer inputs.
```xml

```

Load the supplied MaterialX sample in both GraphEditor and Viewer, then compare the texcoord index values with the other integer inputs. Trace how each application reads and displays the texcoord node, and verify that the values 10 and 100 are preserved rather than reset to 0.

Written by the indexing model from the issue text.