[IronPython] Attempt to update field 'Z' on value type 'Vector3'; value type fields cannot be directly modified

[Jonathan Jacobs]

Hi, me again!

I realise the error seems logical enough, but it isn't right. The 
documentation for Vector3 specifies that the fields X, Y and Z are all 
read/write and indeed C# code has no boggle doing just that. 
Vector3.GetType().IsValueType is true; I guess relying on IsValueType isn't 
enough.

-- 
Jonathan