Clarify that reference/occurrence date are the same thing

[sbfnk]

The documentation / example data sets use date of reference and occurrence interchangeably where it seems they're the same. I'd suggest to either decide on one and use consistently (preferred) or else to clarify at first use in each document that they're the same.

[seabbs]

I think we should shift to using only reference as the standard.

[seabbs]

Closed by @sbfnk in #71