CATS sample changer implementation

Modular implementation, i. e. separated implementation of transfer
and "maintenance" commands, based on Alexandre Gobbo's work for the GenericSampleChanger and
the EMBL SC3 implementation as well as Guifre Cuni's CATS Tango device server

transfer: CatsBrick.py + Cats90.py HO + cats.xml
maintenance: CatsMaintBrick.py + CatsMaint.py HO + catsmaint.xml

Most of the functionality should be implemented and working properly. 
Nevertheless the CATS implementation should still be considered in Beta state and is _NOT_
yet ready for production use because not all errors are handled properly yet.

TO-DO:
- error handling of Tango commands 
- barcode scanning: the barcode can be read during transfers but not as a separate command???
- SampleChangerState sequence buggy
- additional functionality: 
  display of power state, 
  display last CATS error message,
  LN2 regulation commands