Show gain and loss in one color scale legend item

[creilly8]

Description

Closes issue #2374.

The cnv gain and loss legend item data is merged into one legend item in svg.legend. The goal was to recreate the cnv color scale in the mds3 track.

Changes:

1. Increase code encapsulation for the color scale, code reorg
2. Update svg.legend to combine gain and loss legend items into one a parents property
3. Minor update to the disco code to support the merged gain and loss items
4. Minor updates to the matrix interactivity code to act on parents of a legend item to maintain the functionality.

Test

1. Disco example
2. Matrix example

Checklist

Check each task that has been performed or verified to be not applicable.

• Tests: added and/or passed unit and integration tests, or N/A
• Todos: commented or documented, or N/A
• Notable Changes: updated release.txt, prefixed a commit message with "fix:" or "feat:", added to an internal tracking document, or N/A

[xzhou82]

ash matrix cnv legend doesn't show properly