Add Chunked or Segmented file reference

## Feature Request

It would be useful to have a type of `MediaReference` that refers to a contiguous clip that has been "chunked" or "segmented" out over multiple files.

## Description

Much like an ImageSequenceReference(#69) there is a similar kind of single conceptual asset that can be spread over multiple files but rather than containing a single sample (frame) from that file, they contain a subset of the samples that make up the source. These are typically referred to as "chunked" or "segmented" files.

Some common example cases where these may come up include:

- Camera files recorded by RED cameras in both `R3D` and `mov` formats
- Files used in streaming in either fragmented mp4 or `.ts` files

In most cases, this is done to work around file size limitations. Many software packages (like Nuke) have the user point to the first file in the sequence and then implicitly discover the other files in the sequence. It would be nice for OpenTimelineIO to represent this more explicitly to aid in detecting cases where perhaps the last file in a sequence is missing and the clip may be truncated as a result.

#630 is related to this, but it's a bit more targeted at bundling together alternate media references, this would be focussed on media on-disk that is meant to be concatenated to form the full contiguous source.

Start by reading the existing MediaReference and ImageSequenceReference concepts, including related issue #69, and compare the distinction from alternate media references described in #630. Define what a chunked or segmented reference must represent, including contiguous files and detection of a missing final file; the feature is done when this behavior is specified and integrated consistently with the existing media-reference model.

Written by the indexing model from the issue text.