Set Gaudi copyright and license

LHCb and ATLAS have now both approved that the Gaudi code should be copyrighted as

Copyright (C) 1998-2019 CERN for the benefit of the LHCb and ATLAS collaborations

and licensed as Apache2, so we should proceed with adding this to the repository.

Note that as a protection against combining with other code, such as GPL, we stated for ATLAS that The software in this repository is released under the Apache 2.0 license, except where other licenses apply. (https://gitlab.cern.ch/atlas/athena/blob/master/LICENSE). We should discuss if this is still necessary for Gaudi.

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