Add position filtering to SIGMET call

[jlefkoff]

Currently the SIGMET api call returns all SIGMETs in the US. The correct EDST behavior is to only show SIGMETS within 150nm of your sector.

[jlefkoff]

For now use the ARTCC bounding boxes for the math

  "artccBoundariesUrl": "https://data-api.vnas.vatsim.net/Files/ArtccBoundaries.geojson",