WIP fragmented start trimming operational

Tested/debugged fragmented file trimming when the start time is in a
 MovieFragment. The iterator is now properly skipping the pre-fragment
 boxes, deleting the fragments that precede the calculated
 start_movie_fragment, calling the MovieFragment.remove_samples_before_point()
 call, and adjusting the start_movie_fragment and all following fragments
 according to the pre-start byte adjustments to the MovieBox, deleted
 fragments, and the adjusted start_movie_fragment.

Fixed a bug in IsoMovieFragmentHeaderBox. It was missing an implementation of
 update_box_fields(), resulting in an incorrect box size (it needed to account
 for the sequence number field).

Fixed a bug in IsoTrackFragmentHeaderBox. It was also missing an implementation
 of update_box_fields(), resulting in an incorrect box size after modification
 of the base_data_offset field.

Simplified the implementation of IsoTrackRunBox.update_box_fields().

Disabled some debug logging that's no longer needed/useful in
 IsoContainerBox.make_box_for_type() and update_children().

Tested with various MovieBox and MovieFragmentBox start times with
 DLNA fragmented reference content B-AVC_MP4_MP_SD-01.1_1.MP4.