Updating the Tile class getLegend function, so that the colour bar max and min are customisable for different styles.

For "Difference" style, set it to colour the legend from -120 to 120, instead of the default 0 to 150
- so that -ve values show properly.

This fixes issue #1452.
Merge pull request #1453 from PatSunter/fix-analyst-tile-color-legend-difference

Updating the Tile class getLegend function, so that the colour bar max a...