jpmorgen/BigMultiPipe: Feature enhancements and additions

Make outname a callable external function

Add ArgparseHandler and BMPArgparseMixin, which enables an object-oriented approach to constructing adding argparse options

Add cached_pout, which enables pipeline metadata to be written to and re-read from a pickle file

Add bmp_cleanup function

Add code which enables input files to be specified in groups, with each group processed in one sub-process