Re-use of a checkpoint

[sjfleming]

There might be a bug here

CellBender/cellbender/remove_background/run.py

Line 75 in 3efb6b1

'num_failed_attempts', 'checkpoint_filename']

The intention was to have the checkpoint argument ignored, but I may have changed the name of the arg in the past from "checkpoint_filename" to "checkpoint". I don't think it is correctly ignoring that input arg anymore when computing a workflow hash.