Process a list of handwriting samples saved as PNG images in a directory:
(1) Load the image and convert it to black and white with readPNGBinary()
(2) Thin the handwriting to one pixel in width with thinImage()
(3) Run processHandwriting() to split the handwriting into parts called edges and place nodes at the ends of
edges. Then combine edges into component shapes called graphs.
(4) Save the processed document in an RDS file.
(5) Optional. Return a list of the processed documents.
Logical whether to skip documents in input_dir that
caused errors on a previous run. The errors and document names are stored
in output_dir > problems.txt. If this is the first run,
process_batch_list will attempt to process all documents in input_dir.