Contour Layer - support Isobands

[1chandu]

Background

Current Contour layer supports rendering of contour lines (iso-lines), enhance this layer to support iso-bands.

To Do List

• [X] Add label and assign to milestone
• [x] Coding
  • [x] Update Marching Squares utility methods to support iso-bands
  • [x] Update ContourLayer to render iso-bands.
  • [ ] Use interpolation on cell values, instead of 0.5 to calculate intersection points.
• [x] Doc update
• [x] Whats new update
• [x] Test

[1chandu]

Resolved by #2429 and #2434