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

Start by locating the sample-changer rack configuration XML and the existing sample-changer motion setpoints OPI. Trace how the current configuration is loaded and recalculated, then use the acceptance criteria to verify that rack choices can be made per slot and the updated configuration can be reloaded from one OPI.

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