If parameter Duration_Method is set to Klatt the duration
rules from the Klatt book (allen87, chapter 9). This method
requires minimum and inherent durations for each phoneme in the
phoneset. This information is held in the variable
duration_klatt_params. Each member of this list is a
three-tuple, of phone name, inherent duration and minimum duration. An
example for the mrpa phoneset is in lib/klatt_durs.scm.