scanner header creation

[KrisThielemans]

Best to use shared_ptr<const DetectorCoordinateMap> Scanner::get_detector_map_sptr() . However, sadly we run into UCL/STIR#1078.
So, currently we'd have to use a BlocksOnCylindrical scanner, or make the geometry ourselves.

[KrisThielemans]

done in C++, but with a hack.