Detector positions

Add classes to accommodate detector locations using r, alpha, beta.

Todo:
- [ ] Add cluster/station rotation for r, alpha, beta subclasses
- [ ] Allow easier mixing of the various coordinates (detectors using x, y, orientation and r, alpha, beta)
- [ ] Make clear if whether we use true (geodetic) or magnetic North
- [ ] Add tests

@NiekSchultheiss @davidfokkema