Adds read/writing of voxel files to VoxelTree

new features added to VoxelTree
- void writeToFileV2(const char* filename) const;
- void readFromFileV2(const char* filename, bool reset);
- void deleteVoxelAt(float x, float y, float z, float s);
- VoxelNode* getVoxelAt(float x, float y, float z, float s) const;
- void createVoxel(float x, float y, float z, float s, unsigned char red, unsigned char green, unsigned char blue);