Feat/externalize mapcache building

[piellardj]

This PR drastically changes the way the Terrain interacts with the IVoxelMap.

The Terrain now asks for chunks of the map when needed.

The IVoxelMap now only requires one method: getLocalMapData(from: Vector3Like, to: Vector3Like): Promise<ILocalMapData>; that returns the data for a portion of the map.

The ILocalMapData is defined as follows:

/** Compact object storing a portion of the map data  */
interface ILocalMapData {
    /** Compact array storing the voxel data.
     * Each element in the array represent a coordinate in the map and stores the data of the voxel at these coordinates.
     * An element:
     * - should be equal to 0 if there is no voxel at these coordinates
     * - should be equal to the voxel's material id + 1 if there is a voxel at these coordinates
     * 
     * The elements should be ordered by coordinates as follow by Z first, then Y then X.
     * For example, for a portion of the map between (0,0,0) and (2,2,2): (0,0,0) then (1,0,0) then (0,1,0) then (1,1,0) then (0,1,1) then (1,1,1)
    */
    readonly data: Uint16Array;

    /** Should be:
     * - true if there are no voxels in the data
     * - false if there is at least one voxel in the data
     */
    readonly isEmpty: boolean;
}