Add PR review checklist

[harpolea]

A PR review checklist should be added to assist when reviewing pull requests. This should take the form of

• a concise checklist which is posted in the PR itself for the reviewer to tick off. This could be done e.g. by triggering a github action when a reviewer is assigned which comments the checklist in the PR
• an expanded form of the checklist in the TARDIS documentation which explains the points in more detail.

The checklist is not intended to be exhaustive (as different PRs will require different checks), but should give the reviewer a starting point for things to consider and provides a minimum standard which all PRs should meet before they are accepted.