Min/Max fields for HeightMap terrain generation

[JamesTKhan]

Unlike the Perlin Noise generator, there is no way to input custom min/max height values when loading a height map onto a terrain. We should look into adding min/max fields on the HeightMap tab. The height map generator code already supports a max value field and uses an hard coded value (see discussion), but may need extending to add min height.

Perlin with min/max

No min/max

Discussed in #169

[antzGames]

Terraformer.heightMap(terrainComponent).minHeight() looks like cannot be negative and it seems that the minimum is always 0.

Therefore, I have implemented a Min/Max field. If positive then the terrain generates from 0 to max value. If negative, then it inverts the height map. I think of this as a feature. If min/max value set is blank or is zero, it automatically defaults to +100.

[JamesTKhan]

Resolved by #182