BUG: Plane AttributeError: can't set attribute opd when redefining opd property in subclass

[Andy Kee]

Describe the issue:

The Plane amplitude and opd attributes are managed by properties. The Plane constructor calls the property setter method during object initialization (rather than directly setting the "private" attribute value). This works fine until you try to subclass Plane and write a custom amplitude or opd property.

The fix is to directly set the private amplitude and opd attributes in the Plane constructor.