begin
Opens a record for the experiment and returns the id that keys every later call.
Return
the experiment id
Parameters
header
what is known before the first problem runs
resume
when true, attach to an existing unfinished record for the same experiment name and return its id, so a re-run continues where the interrupted one stopped. A finished record is never resumed into; it gets a fresh id, preserving the append semantics under which re-running a completed experiment accumulates alongside it. When false, always a fresh id.