Feat/asynchronous sampling

[piellardj]

This PR:

• changes the interface of the sampleHeightmap() method so that:
  • samples are requested in batches
  • the result can be returned either be synchronously or asynchronously
• changes the interface of the getLocalMapData method so that the result can be returned either be synchronously or asynchronously
• fixes a bug where the generate voxels meshes could sometimes have holes in them
• fixes a bug where the LOD meshes were too shiny compared to the voxels meshes
• fixes a bug where sometimes the junction between LOD and voxels meshes was too visible because of holes
• completely refactors the code:
  • clear separation of the heightmap-related code and the voxelmap-related code
  • decoupling of the map and the voxels mesh generation