SOP/UOP may not be available

[gavanderhoorn]

Depending on how the controller is configured, the UOP and/or SOP IO elements may not be available.

This makes it impossible to use various functions in controller.py:

https://github.com/gavanderhoorn/dominh/blob/d47b6db714feb05d09866fa2893484938f7df320/src/dominh/controller.py#L174-L245

as they make use of those elements.

[gavanderhoorn]

With 9afa99a at least the example doesn't crash any more.