More cluster subclasses and default detector layout around GPS

Add the following subclasses to sapphire.clusters:
- Single station with 2 detectors
- Single station with diamond layout

Default station detector layout now places detectors with GPS at 'center' instead of using the center of mass (detector 2).
Add functions to calculate the center of mass of detectors in a station or that of stations in a cluster. 

Fixes:
- Fixed 508 orientation
- Removed unused keyword
- Add more info about 502 shape change