Issue #820 - Replace customise_metric with `purrr::partial`

Description

This PR closes #820.

Does what it says on the tin. I updated the documentation and tests to reflect the change.

We could (and maybe should in the future) move the customising metrics part of the documentation for score() into a separate vignette (see e.g. https://github.com/epiforecasts/scoringutils/issues/789).

Thanks to @Bisaloo for your help and suggestions here!

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