World.get_node and set_default_node

[S-S-X]

• Added world.set_default_node which sets node returned for locations where nodes have not been placed yet.
• Added world.get_node for better behavior management.

Verify what happens if minetest.get_node({x=0.1,y=0.2,z=0.3}) is called, will it return node at 0,0,0 or will it fail? Remove vector.round if engine fails in that case.

[S-S-X]

Engine get_node returns same node around x -0.5 - 0.5, y -0.5 - 0.5, z -0.5 - 0.5 Rounding should work without problems.