Externals Sync to TC (2017.03.28.)

I tried to thoroughly get all the externals in sync with how they were set up from SVN just before TC was closed. This should bring CLHEP and G4 to the same version (this is what was complained about mostly), but also FastJet and FastJetContrib to slightly older tags that were the ones used in 21.0.X from SVN.

The success of the changes will of course only be fully tested in a merge request to athena's 21.0 branch, but the CI test of this repository should already be a useful check. (I did test the build locally of course.)