Implement pl$mean_horizontal() function

[george-wood]

Proposed in #854.

[george-wood]

• I reverted a few changes to unrelated Rd files.
• I also added pl$mean_horizontal() to the seealso of pl$mean() and removed the related TODO.
• I ran task test-snapshot-accept following the R CMD failure.

[eitsupi]

Thanks!

I have updated the Rd files and test snapshots.

[etiennebacher]

Nice, thank you @george-wood 👍

There were a couple of minor things to improve in the docs, see the followup here: #960

[george-wood]

@etiennebacher Good improvements. I used NaN and -Inf in the tests, but not the examples. It's good to clearly show this behavior (especially for NaN/null) in #960.