Avoid calculation of file duration for NaT end entries

[hagenw]

When having NaT as end entries, e.g. when using a filewise index, auglib.Augment.augment() will need to calculate the duration of the files. This is in principle not nessecary as this is done anyway during augmentation and the information can be easily returned when setting self.keep_nat to False for the underlying audinterface.Process object. But we cannot easily do this when calling super.__init__() as this would affect other methods like auglib.Augment.process_file(). So most likely we need a bigger refactor of the code of auglib.Augment before we can achieve the goal of this issue.