tools/fio_jsonplus2csv: accommodate multiple lat measurements

tools/fio_jsonplus2csv: accommodate multiple lat measurements

Add some intelligence to this script so that it works for any of
submission, completion, and total latency whenever they are present. The
CSV data format is changed to accommodate this.

While we're here also do the following:

add a way to generate optional debug output
add validate option that compares generated CSV data with the original
json+ data
fix style issues identified by pylint3
update documentation

Signed-off-by: Vincent Fu <vincent.fu@wdc.com>