refactor _run_task

[satra]

_run_task is currently a Task method and therefore has access to self and thus checksum and output_dir. however, the returns of these functions can be different inside _run_task relative to the original inputs of the task because of modifications to inputs that are done before calling run_task. it would be good if run task didn't have access to these fields.

one thought process is to turn the method into a staticmethod or callable function that receives inputs and output fields as arguments.