Add functionality to the unified_rw_reporting parameter to output

Add functionality to the unified_rw_reporting parameter to output
separate and mixed stats when set to 'both' or 2.

To achieve this, the stats are gathered on a per/data direction
(read/write/etc) basis and then summed into the mixed stats when
generating output.

This functionality also allows setting unified_rw_reporting to
'none' or 'mixed' instead of 0 or 1 respectively.

Signed-off-by: Brandon Paupore <brandon.paupore@wdc.com>
Co-authored-by: Jeff Lien <jeff.lien@wdc.com>