Feature Request: Limit based on altitude

[rcarsey1]

Similar to the --max-range option that limits dump1090 to positions within a certain range of a specified point. I'd like there to be a --max-altitude as well which does what you would expect.

My goal is to enhance low-altitude coverage at my local general aviation airport. I have an expensive LTE hotspot link to the internet as my airport does not have internet service of any kind (the same could be said about most airports in Alaska too!). I do not care about aircraft above 4000ft in my area because they are being picked up by 100 other receivers in my area (New York).

I'm happy to give a stab at programming this myself, but seems like it should be an easy feature to incorporate.