 OSLShaderUI : Hide spline parameters in NodeGraph
 OSLShaderUI : Hide spline parameters in NodeGraph

There's nothing sensible to connect them to. This also fixes the following error when trying to promote a spline parameter :

```
RuntimeError: Exception : Cannot set value for plug "out.interpolation" except during computation.
```

The problem here was that when promoting via a BoxIn node (the code path if there's a nodule for the parameter), an output SplinePlug was created, and in the constructor `setValue()` was called.

boberfly/gaffer,boberfly/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,hradec/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,boberfly/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,hradec/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,hradec/gaffer,andrewkaufman/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,hradec/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,hradec/gaffer,appleseedhq/gaffer,boberfly/gaffer