Revise data model for double routes

[martian-a]

The Nordic Countries map features a double route with each option having different ferry requirements. They're currently represented by nesting the asset requirements within the colour expression.
Change so that colour elements no longer also represent a route option. Instead, use an explicit option element.

Before:

• route
  -- colour
  ---- asset
  -- colour
  ---- asset

After:

• route
  -- option
  ---- colour
  ---- asset
  -- option
  ---- colour
  ---- asset