Rewrite fHits() in arc_summary.py with SI units

Rewrite fHits() in arc_summary.py with SI units

Complete rewrite of fHits(). Move units from non-standard English
abbreviations to SI units, thereby avoiding confusion because of
"long scale" and "short scale" numbers. Remove unused parameter
"Decimal". Add function string. Aim to confirm to PEP8.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Scot W. Stevenson <scot.stevenson@gmail.com>
Closes #6815