initial commit
as11: add Present suffix to Open/ClosedCaptions properties
op1a: add support for vc-3
avid: write subsampled widths for dvcprohd

Avid uses the horizontally subsampled widths in the StoredWidth
property for DVCProHD. We follow this example to avoid potential
issues caused by differences with Avid generated files.
mxf reader: use updated partition reading methods

Also results in the code continuing to process the header metadata
if not all the partitions could be read. It will however fail when
trying to index the essence data
mxf reader: rearrange mxf reader class hierarchy

MXFClipReader renamed MXFReader and sits at the top of the hierarchy
MXFReader renamed MXFFileReader
mxf_reader: rename parent_reader variable to file_reader
mxf reader: rename reader variable to file_reader
mxf reader: fix comment's reference to a method name
mxf reader: check samples rates in max precharge/rollout method

The code currently assumes that when an external reader has non-zero
precharge or rollout that the external reader's sample rate equals
the parent reader's sample rate
mxf reader: added mxf group reader

The MXFGroupReader can be used to read a group of related
MXF OP-Atom files for example
mxf reader: create abstract MXFTrackReader and subclass the existing class

Also, the compare_group_track_reader function in MXFGroupReader
uses the track's TrackInfo class to compare the material package uids
mxf reader: warn if group reader has no members
mxf reader: move misc package info up to MXFReader
mxf reader: set group package info if all members agree
mxf reader: add const qualifiers to various methods
mxf reader: make class methods const
mxf reader: change warn log to error log for consistency
mxf reader: created single impl of GetSampleSequence(rate)

The call in MXFFileReader was reversed to get the same semantics
as the calls in MXFGroupReader
mxf reader: handle negative positions/durations in conversions