359: convert get_pit() to S3

[sbfnk]

Description

This PR is an intermediate step towards addressing #359 as mentioned in #359 (comment).

It turns get_pit() into an S3 function so an appropriate method can be chosen according to how the predictive distribution is represented.

Checklist

• My PR is based on a package issue and I have explicitly linked it.
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[seabbs]

LGTM. Much nicer.