AthGeneration FastJet Handling Update, 1.0 branch (2020.01.07.)

This is to (try to) handle the requirement from AthGeneration, that it would like to use FastJet and FastJetContrib from LCG_88. Instead of building them as part of AthGenerationExternals.

This turned out to be a bit more involved than I first thought. 😦 After removing the build of FastJet and FastJetContrib from the AthGenerationExternals project, I had to realise that HepMCAnalysis was using FastJet in a very hardcoded way. So I had to teach HepMCAnalysis (and in consequence had to update all projects) how to build itself when FastJet is used from LCG. (Or somewhere else. Important part being just that it's not being built as part of the current project.) While I was at it, I also taught FastJetContrib how to build itself against an "external" FastJet installation. If we should ever decide to have such a setup...

Finally, I also had to move and update FindFastJetContrib.cmake. Since so far it was only installed into the release when we built FastJetContrib ourselves. But now we need to become able to use FJContrib even when we don't build it ourselves. And in any case, now that LCG provides this package, it makes sense to house this module as part of AtlasLCG.

Tagging @abuckley, @ewelina, and @cgutscho.