'Create a new Duration object.
= ERROR CONDITIONS
- If the input frame is not in the allowed list, an error is thrown.
= INPUT VARIABLES
- frame    The frame of the duration.  Must be \'ET\' or \'UTC\'
- seconds  The number of seconds in the Duration.'

def __init__(self, frame, seconds):

    if (frame not in self.allowed):
        msg = (u"Input    frame    '%s'    is    not    one    of    the    supported    frames    of    %s" % (frame, str(self.allowed)))
        raise ValueError(msg)
    self._frame = frame
    self._seconds = seconds
