Classification lab record id - add comments / helper methods

I was using this for the 1st time in SACGF/variantgrid_private#3836 and found a few things not immediately obvious

**Document various "id" strings**

Hard to work out what “record_id” and “id”, “id_str” etc are and how they fit together

**id_str helper function**

There is a helper function to deconstruct the id_str - `ClassificationRef.parse_id_str`

Would be nice to have one to make it, ie Helper to make `id_str = f”{lab.group_name}/{lab_record_id}”`

James says:

> I think it's org-group-name/lab-group-name/unique-Id-within-lab