- Convert `isodate.Duration` instances to `datetime.timedelta`
  in `SegmentTemplate.format_media()` in order to be able to compare
  with `datetime.datetime` in the timeline generator
- Keep `isodate.Duration` or `datetime.timedelta` instances
  in `SegmentTemplate.segment_numbers()` before subtracting from
  `since_start`, as `isodate.Duration()` can't be converted to seconds
  without a reference `datetime.datetime`
- Replace all `isodate.Duration()` attribute default values with
  `datetime.timedelta()` for simplicity/performance reasons
- Set the 3 seconds default value of `MPD.suggestedPresentationDelay`
  on the attribute's default definition instead of using constants
  in the `SegmentTemplate` methods