Modernize GaudiGSL
- move singleton declaration out of .h into .cpp
- prefer struct over class if only public data
- make binary operators symmetric wrt. promotion by making them friend functions
- use std::tie to implement operator<
- prefer inheritance from extend_interfaces over virtual inheritance from IFace
- prefer implicit destructors
- prefer implicit copy constructors
- prefer boost::variant over equivalent 'raw' code
- prefer std::vector over std::unique_ptr<T[]>
- prefer std::unique_ptr with custom deleter over explicit code
- prefer CommonMessaging
- prefer inherited constructors
- add final
- avoid unnecessary temporay vectors
- prefer switch over cascading if statements
- prefer cmath over math.c, cstdio over stdio.h
- prefer '\n' over std::endl
- do not call exit(0) when it is not necessary
- add support for
Gaudi::Property<GaudiMath::Interpolation::Type>