Implement FlatCityBuf output

[ymoisan]

FlatCityBuf (FCB) output is in my opinion a must for cloud native ops. Not only does the format allow stuffing everything in one file -- compared to a number of per feature CityJSONSeq text files plus a separate metadata.city.json file -- but I have also tested how much more efficient it is to create 3DTiles from FCB compared to CityJSONSeq

Roofer outputting FCB would directly pipe into Tyler, resulting in a complete pipeline to generate 3DTiles 1.1 from reconstructed buildings.

Are there plans for FCB output already ? We can help.

[Ylannl]

Yes this is something that I would like to see, in fact it is part of the reason why we developed FCB. I just didn't get to implementing it in roofer yet. If you would like to work on this, I'ld more than happy to review a PR.

[ymoisan]

#149

This is a rough attempt. It works with a small test set. It's a bit clunky (links to Rust) but AFAICT it works.