Add Position closest to beam to all functors test

Added position of state closest to beam as per discussion https://mattermost.web.cern.ch/lhcb/pl/fka4kzeqj3y7tfkohq3aqwwbgr involving @graven @mstahl

Requires LHCb!3885 (merged) (for completeness; this is already merged).

I am not sure it's very useful as it does the same as REFERENCEPOINT, but it's an example of something that should work. Hence good as a test, not necessarily as an example.

Superseded by !793 (merged) .