Add CLHEP to AnalysisBaseExternals
In !998 (closed) we tried to introduce the BootstrapGenerator package recommended by the ATLAS Statistics Forum into AnalysisBaseExternals, in order to write a CP algorithm for it and introduce it into standard analysis code in release 22. The MR was closed due to concerns over licensing.
Instead, we now would like to make our own bootstrapping algorithm, based this time on CLHEP.