intervals extracts the `interval' attribute if it exists.
The attribute `intervals' is set automatically by the secr function
join.
sessionlabels provides session names for the primary sessions encoded
in a ``single-session'' capthist object (e.g., the result of join)
that has an intervals attribute. The names are used by some summary functions
in the package openCR (M. Efford unpubl.) (m.array, JS.counts).
The function session has a different purpose: labelling
sessions in a multi-session capthist object. However, session
names of multi-session input are used automatically by join to construct the
sessionlabels attribute of the resulting single-session object.