Improve ore generation to better match recent Minetest versions

[GreenDirkfried]

This fits the moreores ore generation to the default ore generation.

1. Three layers (two underground and one high air/ space) instead of just one layer.
2. Don't use chunk size as clust size anymore. Clust sizes are usually just 3 nodes and not the whole area ("chunk"), where the ores are generated.
3. Adjust more default values.
   3.1. Mithril is now generated below diamond (!). Note that there was a change in the minetest update 5.0.0, where most ore generation was shifted to much lower altitude (shifting diamond generation altitude below mithril generation altitude).
   3.2. The mithril ores are now also grouped together and not just found as a single node in one chunk.
   3.3. I tried to retain the same overall ore density (for the deep layer).

[Calinou]

Thanks!