Read trees are not in sync

[kreczko]

One big issue with reading multiple trees is that they all have to be read independently, e.g.

TreeChain(
    treeName,
    self.input_files,
    cache=True,
    events=self.nevents,
)

or

uproot.iterate(
    self.input_files,
    treeName,
    entrysteps=self._batch_size,
)

In order to safeguard ourselves against such behaviour the easiest way is to switch to file-based reading: one file at a time. For vectorized processing, this means switching to uproot.open + for each tree: lazyarrays(entrysteps=batch_size)