Mebristo/refactor run id types

[mebristo]

Make the following changes to command line args for himl-tensorboard and himl-download:

• Replace ArgumentParser with parametrized class
• replace run_id and run_recovery_id with --run which can be one or more of either type
• keep experiment (name) arg. If neither --run nor --experiment is provided, defaults to searching for a most_recent_run file

update "get_run" functions in line with new command line args:

• one function for getting Run objects from run ids/ run recovery ids
• separate function for getting Run from experiment

Side change:
Update docstring & signature for download_checkpoint to highlight that it can be called to download one checkpoint file, as well as a folder.