consider naming reference_date as a possible standard task id field name

[elray1]

Looking at this page: https://hubdocs.readthedocs.io/en/latest/user-guide/tasks.html#usage-of-task-id-variables

We describe origin_date there, but subsequent discussions in some hubs have led to the also-reasonable choice of reference_date for the name of this variable.

[mzorn-58]

see #126