Review letters used for command line arguments

The letters used for one-letter command line arguments seems to have been assigned in an ad hoc fashion partly based on common practice. However, as the number of options slowly grow we might want to review, and possibly change, some of these before the package is getting used in wider circles. Currently we have:

  -h, --help
  -l {debug,info,warning,error,critical}, --log-level {debug,info,warning,error,critical}
  -v, --verbose
  -e, --deactivate-error-tracking
  -d DASK_SCHEDULER, --dask-scheduler DASK_SCHEDULER
  -k, --keep-open
  -s, --sliced-mode
  -i, --iterative-storage
  -o OUTPUT_TEMPLATE, --output OUTPUT_TEMPLATE
  -x INDEX, --index INDEX

And there are plans (cf. #228) for an argument for replacing individual index definition elements, and for specifying index_definition file.