Switch all projects to LCG_97 and atlasexternals-2.0.62 (2020.04.03.)

Switch all projects to:

• LCG_97;
• tdaq-09-00-00;
• tdaq-common-04-00-00;
• atlasexternals-2.0.62.

as discussed on the GRC mailing list. Once this is merged we need to separately switch our Python3 nightly to the corresponding versions (tdaq-09-00-01, tdaq-common-04-00-01).

Changes necessary for LCG_97 compatibility:

• Updated xAODDataSource to fix a coding error that was masked with previous ROOT versions;
• Added HepMC3 to the runtime setup in Sherpa_i, since Sherpa now needs it;
• Removed the explicit setting of external versions in the projects.

cc @akraszna @emoyse @wlampl