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>