Rename `metrics` argument in `correlation()` and fix default behaviour

[nikosbosse]

As @seabbs mentioned in #588, correlation() has an argument metrics.

• As outlined in Create documentation/vignette for the naming scheme of our functions #520 this argument should be renamed to rules
• The default gets immediately overwritten in the next lines. Either we should make get_score_names() the default, or we should keep the default NULL and use match.arg() in the code.

[seabbs]

This is now out of date due to discussions in #520, #600 and elsewhere