Remove requirement for entity identifier in event log to be titled 'patient'

[Bergam0t]

This sprung out of the original intended use of the package in healthcare contexts, but wider application means 'patient' isn't always particularly intuitive to use as the identifier (and even within healthcare contexts this isn't actually always the unit we're tracking).

To avoid breaking, ideally should allow either 'entity' or 'patient'.

(in longer run, could allow mapping to custom column names, but that may be a separate thing to explore)