Version 4.1.20 Improved metrics parsing

Resolved parsing the --metrics options, so you can either add one --metrics flag and list all your options, e.g.

PhiSpy.py --metrics shannon_slope gc_skew

or provide each one with its own flad, e.g.:

PhiSpy.py --metrics shannon_slope --metrics gc_skew

This is a combination of using argparse append and then iterating the list.