Add bulk processing to analysis spans

The process_file implementation invoked the span processing function for
each and every frame. This commit adds a bulk processing function to the
spans as well as a query for how much data the span wants.
Add choice to downmix display to stereo

Historically, we've either downmixed to mono, or left display as-is.
Now, we're additionally allowing a stereo display, which downmixes
higher channel counts and leaves mono as-is.
Bumping version to 0.2.30
Merge branch 'release/0.2.30' into develop