Flatten the GENIE event record in flat CAF

[kjplows]

Description

Please provide a detailed description of the changes this pull request introduces. If available, also link to a docdb link where the issue/change have been presented on/discussed.

This PR changes the way the GENIE EventRecord is stored in flat CAFs. Instead of storing the entire genie::EventRecord (as is done in the regular CAF), the information of the EventRecord is saved as C-style arrays. The convention follows that of the GENIE rootracker format.

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• Does this PR affect CAF data format? If so, please assign a CAF maintainer as additional reviewer.
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[jedori0228]

Thanks @kjplows ! Just tested this PR and seeing the flattened GENIE trees in the flatcaf output.