Modernize LHCbKernel

• do not provide default arguments in derived functions, as the caller only (has) to see the base class definition,
• prefer decltype(auto) over std::result_of<...>::type
• make binary operators friend functions to make them left-right symmetric wrt. implicit conversions
• prefer cmath over math.h
• qualify single-argument c'tor as explicit
• fully qualify enum values