use last recent airplane position to calculate its ground position

Inspired from <a href="https://groups.google.com/forum/#!msg/dump1090/svjOfcdn43Y/CkBAIf3a_PUJ">this</a>.

When a plane is on the ground and we have received its location recently when it was in air, then that position is used in calculating the DF18 ground position.

This is especially useful for when the user does not provide the receiver location, or while one is continuously moving.

Note that this was not tested on real aircraft yet, since I do not get these types of messages locally very much. However, this also does not seem to break the existing code either, so it should be safe, even if it cannot be tested very much.