Bugs in HeightTile and HeightTileTest

[wltrr]

Describe the bug When implementing a new DemProvider, I found some bugs in HeightTile and HeightTileTest:

• In HeightTile, the precision field is in fact for the inverse precision. In the HeightTileTest, 1e-6 was used as precision, leading to an ineffective test.
• In the getHeight method of HeightTile, the relative coordinates of the requested coordinates are computed using Math.abs(). This means that the HeightTile that is meant for (0,10) x (0,10) would return a height for (-10, 10) x (-10,10). I don't think this was desired behaviour.

On the other hand, the HeightTileTest had some tests that in my opinion should expect an Exception. I am not sure what the original intention of these tests were so maybe it would be good to double check that.