Remove the `across` argument in `summarise_scores()`

[nikosbosse]

As discussed in #791: should we remove the across argument in summarise_scores()?

The idea is to be more opiniated about the happy path that users should take.

From #791:

For example, in summarize_score(), the by and across arguments are redundant, which is confusing for the user, and forces the developers to jump through extra input checking hoops. Since the transformation of one to the other, I would recommend being more opinionated, and having a single, well-documented option.

I guess democracy has spoken 🥲

[seabbs]

This was something you really liked @nikosbosse and if that is the case I think you can push back on knowing the work better than us.

The alternative I see is creating a little function that maps the current across input into something that works in by but that might be a bit tricky

[nikosbosse]

I liked it, but I guess sometimes you need to kill your darlings 🤷.

Surprisingly, I haven't used it that much as I expected.