Investigate negative year difference

[lizgzil]

We find that policy document year minus reference year is sometimes negative, this shouldn't be the case. Investigate the circumstances in which this happens to understand whether there is a bug.

I think there are 2 places where errors could mean negative years :

1. the policy document year metadata has been populated wrongly (not sure how the scraper does this)
2. that the reference was not fuzzy matched correctly (which we know happens with 2% of the time, especially for generic titles)