Polish bam_split

Unit test expand_format_string
Change from output_prefix to output format string
Make usage write to stdout when not an error.
Make stdout usage appear if no arguments given.
Make unaccounted reads an option and whine if found and not specified.
Filter alternate @RG lines from the files they’re not in.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>