Sort of a duplicate of #52 but rather than re-doing how slopes and palettes are handled, just allow different sets of palettes to be defined at different slants.

Example

slant:
  - threshold: 2
    palette:
      - "..." # Areas where: {2 > gradient > 9}
      - "..."
      - "..."
  - threshold: 9
    palette:
      - "..." # Areas where {gradient > 9}
      - "..."
      - "..."

I definitely prefer this config to #52. I'll probably close #52 in favor of this.

Has been implemented in v6 branch