surface position reporting decode

[9nut]

Hi,

for your review.

I've added the decode for surface postion messages (DF=17 subtype 5-8), with these caveats:

1. i'm not entirely sure about the math -- i can't say i understand everything i read in the bits and pieces of the spec that i've been able to find online; but i've tested it at SeaTac and visually verified the correctness.
2. i added the functions for surface position calculations by copying and modifying versions you already had rather than trying to merge the logic or refactor into same functions. if the math is correct, i can merge them with perhaps a flag to signify surface or airborne position.

Thanks, -Skip

[wizardishungry]

You should probably remove the map centering on SEATAC from the PR :stuck_out_tongue:

[antirez]

Thanks, this is something I wanted to do soon or later, so very happy to see this code. I'll review later today hopefully. I moved in the new house and finally I've a new dump1090 installation working so I'll start to merge stuff in the free time again.