Set peak time to zero in GetEi if no peak is found. Refs #6782

Also set initial value of Tzero to 0.0 so that if ei is fixed, this is
the set to 0 automatically.
GetEi2 throws if ei not fixed & peaks are not found.

Also added extra tests around these scenarios. Refs #6782
Merge remote-tracking branch 'origin/feature/6782_getei_peaktime_error'