benchtests: improve argument parsing through argparse library
The argparse library is used on compare_bench script to improve command line
argument parsing. The 'schema validation file' is now optional, reducing by
one the number of required parameters.
* benchtests/scripts/compare_bench.py (__main__): use the argparse
library to improve command line parsing.
(__main__): make schema file as optional parameter (--schema),
defaulting to benchtests/scripts/benchout.schema.json.
(main): move out of the parsing stuff to __main_ and leave it
only as caller of main comparison functions.