Add Davix, 1.0 branch (2018.04.19.)
This is to address ATLINFR-2281.
The update adds a new package, which builds Davix as part of an externals project. It also updates the ROOT build configuration to make use of this standalone Davix library instead of building one internally. Finally, the update extends both AnalysisBaseExternals and AthAnalysisExternals with this new package.