Follow LinksByKey changes in LHCb!3923
-use LinksByKey::link instead of LinksByKey::addReference
and some additional cleanup:
- prefer Gaudi::Property, and use update handlers instead of code in initialize
- prefer inheritance from extends -`prefer getting LinksByKey as argument in operator() for Gaudi::Functional algorithms
- for non-functional algorithms/tools, prefer get<> and getIfExists<> over direct use of SmartDataPtr (even though data handles would be better)
- prefer range-based loops
- add some const