Rework summarise_scores() and pairwise_comparison()

[nikosbosse]

This PR

• adds functionality to get, store, and restore attributes to scoringutils objects. These need to be handled manually, as attributes often don't survive certain data.table operations. This was started here, but is not yet done consistently everywhere
• makes some minor improvements (changes? :P ) to internal check functions
• splits an new function add_pairwise_comparison() away from summarise_scores(), such that the two operations now happen in 2 independent steps.
• Modify add_coverage() such that it also works after summarising scores

Thinking:

• add_pairwise_comparison() now works on an unsummarised_scores attribute. This means that you can't do it twice at the moment. Also the output column is just relative_skill, not relative_skill_myscore, so it would be overridden anyway. Are we fine with this?

To do:

• need to update documentation to reflect that this now happens on the unsummarised_scores attribute --> meaning that you can't do it twice at the moment.

[seabbs]

LGTM