parse_args
=================================================

.. py:function:: petpal.cli.cli_reference_tissue_models.parse_args()

   Defines and parses command-line arguments.

   This function creates an ArgumentParser object, defines command-line arguments using that object,
   and then calls its parse_args method and returns the results.

   The function defines a set of sub-commands (srtm, frtm, mrtm, mrtm2, mrtm-original), each of which has a different
   set of requiring arguments. Each sub-command represents a different analysis method and requires different inputs.

   :returns: *argparse.Namespace* -- An object representing the command-line arguments. The object's attributes are the
             various command-line arguments, where the attribute name matches the option string.

   :raises argparse.ArgumentError: If the argument is already defined.
   :raises argparse.ArgumentTypeError: If the argument type conversion function raises this exception.
   :raises SystemExit: If invalid arguments are passed or `-h`/`--help` is chosen.