If parameter Duration_Method is set to Averages
then segmental durations are set to their averages. The variable
phoneme_durations should be an a-list of phones and averages
in seconds. The file lib/mrpa_durs.scm has an example for
the mrpa phoneset.
If a segment is found that does not appear in the list a default duration of 0.1 seconds is assigned, and a warning message generated.