Avoid `count_nonzero` calls

[pfackeldey]

We should be able to purely load data with this trick:

import dask_awkward as dak

def load_columns(events, columns):
  for col in columns:
    _ = events[col]
  return events

dak.map_partitions(load_columns, events, [("Jet", "pt"), ("Jet", "eta"), ...])

This way we avoid using unnecessary ak.count_nonzero calls in materialize_branches.ipynb

[alexander-held]

This might also help keep memory requirements low, looks like a good setup to try out.