added global conventions attribute of UGRID-0.9

Fixes https://github.com/SciTools/iris/issues/1429 by adding a Conventions attribute of UGRID-0.9

```
// global attributes:
		:Conventions = "UGRID-0.9" ;
```

This was done using:
```
ncatted -O -h -a Conventions,global,o,c,'UGRID-0.9' 21_triangle_example.nc
```