update Z statistics, ntuplizer produces merged ntuples

[jpata]

• [x] ntuplizer merges files_per_job input files to reduce load on filesystem
• [x] switch to consistent sample names, always keep the original p8_*.cmd path, there is no "QCD" sample
• [x] move final merging and drop step from validate.ipynb notebook to a standalone script (mergeFiles.py)
• [x] delete outdated dataset.yaml files

The final ntuples are here:

2.8G    /local/joosep/ml-tau-en-reg/ntuples/20240625_all_2M_merged
2.8G    /scratch/persistent/joosep/ml-tau/20240625_all_2M_merged/

The statistics are as follows, in jets:

qq: N=3523618, Ntrain=2818894 Ntest=704724
zh: N=2133620, Ntrain=1706896 Ntest=426724
z:  N=1828006, Ntrain=1462404 Ntest=365602