Sample Changer: Allow rack to be changed from OPI

As a SANS instrument scientist I don't want to have to edit an XML file when I change the rack configuration. I would like to be able to select which rack is present in each slot from a list and reload the configuration from a single OPI. This limits the amount of setpoints available to the user in the motion set points OPI to only those which are available.

## Acceptance criteria
- [ ] The type of rack in each slot can be selected from an OPI
- [ ] This OPI can also re-calculate and reload the new configuration