Modernize LoKiCore

* remove redundant explicit destructors
* replace some 'virtual' qualifiers with 'override' qualifiers
* add 'final' qualifiers
* if possible replace constructors with forwarding to
  base class constructors
* tweak move assignment / move construction
* prefer std::unique_ptr over raw pointer + explicit delete
* use 'auto' where appropriate
* implement {Min,Max,AbsMin,AbsMax} as special case of Extremum
* implement {Min,Max,AbsMin,AbsMax}Element as special case of ExtremeElement