Take account of drift

[tdealtry]

Currently, we use the CI output to check whether parameters are changing, and if they are changing by a small amount (say less than 1%), we typically say that is ok, merge the PR, then issue a new tagged release

What we are missing is a way to ensure we're not drifting. Say the number of hits in the OD changes by +0.5% in 5 different PRs that are separated from each other by releases. What we see in the CI is a +0.5% change, which is valid, so we accept it. What we miss is that the number of OD hits has increased by 2.5% over the course of these PRs/releases.

To account for this, we need to do something like comparing new reference files with multiple previous reference files (e.g. from previous release, release -5, release -10, etc.)
Note that this comparison gets less trivial the further you go back, as config options can & will change