Zero copy merge (WIP)

[Eug9]

This Pull request:

Add a draft implementation of zero-copy merge of RNTuples. @jblomer

Changes or fixes:

Checklist:

• [X] tested changes locally
• [ ] updated the docs (if necessary)

This PR fixes #

[phsft-bot]

Can one of the admins verify this patch?

[jalopezg-git]

Nice work, @Eug9! (review comments coming soon). :slightly_smiling_face: I hope that you enjoyed your internship at CERN!