Add --output-format option for TTree and RNTuple output

[deimantae]

Added a new command-line option to select the output format, either TTree or RNTuple. The default output format remains TTree, so the behavior of the code does not change. When --output-format rntuple is specified, the output is written as an RNTuple using ROOT.RDF.RSnapshotOptions.

fccanalysis run <analysis.py> --output-format rntuple

[jmcarcell]

This is a good idea. In podio it's also lower case rntuple what we recommend: https://github.com/AIDASoft/podio/blob/master/doc/reading_writing.md. I just realised that in k4FWCore we use RNTuple with the casing, which is not consistent, so I'll change that. Note that for this to work a recent version of ROOT is needed (I think when RNTuple was taken out of ROOT::Experimental so 6.36 maybe and 6.38 works). Also some of the tooling around RNTuple (like hadd) prints messages saying that it is experimental with RNTuple, even with 6.40.02.