[WIP] Multi-LD

[abought]

Ticket

#213

Purpose

Allow users to request more than one LD reference variant within the same plot. This is useful for regions with more than one potentially independent signal.

This should also serve as a proof-of-concept for using widgets to encapsulate complex calculation logic, like visualization options that affect many fields at once, or legends that should rewrite their own definition to reflect the currently selected variants:

• Layout rewrites (dynamically updating panel legends to show color schemes for 1..4 variants)
  • Should respect alternate layouts, eg if the display_options widget has overridden default behavior, then only rewrite legend when the legend is showing LD
• Clearing old LD reference variants as user scrolls to a new region (region_changed event)
• Tooltips should now show best-match ld variant, new add variant links etc

Summary of changes

• Allow scalable parameters to request/receive more than one field in the same custom scaling function
• Legends now support new "shape: ribbon" option describing LD palette
• LDServer adapter now allows requesting more than one LDRefvar, and provides additional fields of info, eg which refvar is the best match
• Allow scatter plots to configure stroke color (one value for all points)- move from SCSS to d3 root element
• New toolbar button that encapsulates layer-layout rewrite logic when multiple ld refvars are selected
• Identify best way to set multiple ld refvars